vsp
DESCRIPTION
Cryogenic distillationTRANSCRIPT
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TABLE OF CONTENTS
1. ABOUT THE ORGANIZATION
2. AIR SEPARATION PLANT
3. DESCRIPTION OF ASU I, II AND III
4. DISTILLATION
5. CRYOGENIC DISTILLATION
6. DEVELOPMANT OF DISTILLATION
7. DISTILLATION DESIGN
8. CONCLUSION
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1.0 ABOUT THE ORGANIZATION
Steel comprises one of the most important inputs in all sectors of economy. Steel
industry is both a basic and a core industry. The economy of any nation depends on
a strong base of iron and steel industry in that nation. Iron and steel making, as India
has known craft for a long time, the growth of steel industry in India can be
conveniently studied by dividing the period into pre and post independent era. By
1950, the total installed capacity for ingot steel production was 1.5 million tonnes
per year. The capacity increased by 11 folds to about 16 million tonnes by nineties.
1.1 TYPES OF STEEL PLANTS
Presently in India steel Products are being produced from 4 different sources,
namely:
Integrated steel plants
Mini Steel Plants
Re-rolling Mills
Alloy and
Special steel plants
In integrated steel plants, naturally occurring raw materials are processed into
finished products in various stages. These plants are highly capital intensive. It
needs approximately Rs. 2500 crores of money to establishment 1 million tonnes
per year steel plant.
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1.2 VISAKHAPATNAM STEEL PLANT (VSP)
Visakhapatnam Steel Plant (VSP), the first coast based Steel Plant of India is located
16 KM South West of city of Destiny i.e. Visakhapatnam. Bestowed with modern
technologies, VSP has an installed capacity of 3 million Tonnes per annum of Liquid
Steel and 2.656 million Tonnes of saleable steel. At VSP there is emphasis on
total automation, seamless integration and efficient up gradation, which result in
wide range of long and structural products to meet stringent demands of discerning
customers within India and abroad. VSP products meet exacting International
Quality Standards such as JIS, DIN, BIS, BS etc.
VSP has become the first integrated Steel Plant in the country to be certified to all
the three international standards for quality (ISO-9001), for Environment
Management (ISO-14001) & for Occupational Health & Safety (OHSAS-18001).
The certificate covers quality systems of all Operational, Maintenance and Service
units besides Purchase systems, Training and Marketing functions spreading over
4 Regional Marketing Offices, 24 branch offices and stock yards located all over the
country.
VSP by successfully installing & operating efficiently Rs. 460 crores worth of
Pollution Control and Environment Control Equipment and converting the barren
landscape by planting more than 3 million plants has made the Steel Plant, Steel
Township and surrounding areas into a heaven of lush greenery. This has made
Steel Township a greener, cleaner and cooler place, which can boast of 3 to 4° C
lesser temperature even in the peak summer compared to Visakhapatnam City.
VSP exports Quality Pig Iron & Steel products' to Sri Lanka, Myanmar, Nepal,
Middle East, USA, China and South East Asia. RINL-VSP was awarded "Star
Trading House" status during 1997-2000. Having established a fairly dependable
export market, VSP plans to make a continuous presence in the export market.
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Having a total manpower of about 16,600 VSP has envisaged a labour productivity
of 265 Tonnes per man year of Liquid Steel. This plant has many modern
technological features, some of them for first time in the country. Among these are:
• 7 Meter tall coke ovens.
• Dry quenching of coke.
• One ground blending of sinter base mix.
• Conveyer charging and bell less top for blast furnace.
• Cast house slag granulation for Blast furnace.
• 100% continuous casting of liquid steel.
• Gas Expansion turbine for power generation utilizing blast furnace top gas
pressure.
• Extensive treatment facilities of effluents for ensuring proper environmental
protection.
• Sophisticated high speed and high production rolling mills.
The soviet design organization, GIPROMEZ designed the coke oven and coal
chemical plant, sinter plant and blast furnace. MECON of Ranchi designed the seven
–meter tall coke oven batteries with dry quenching. The remaining facilities have
been designed by DASTUR & CO, who is the principal consultants for VSP.
1.3 MAJOR PRODUCTION FACILITIES OF VSP
• 3 coke oven batteries of 67 ovens each having 41.6 cu.m volumes.
• 2 sinter machines of 312 sq.m area
• 2 Blast furnaces of 3200 cu.m useful volume
• Steel Melt shop with three LD converters of 150T capacity
• Light and Medium Merchant Mill of 710.000 tonnes per year capacity
• Wire Rod Mill of 850.000 tonnes per year capacity
• Captive power plant with a total generating capacity of 280 MW
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• Air separation plant for production of liquid Oxygen and Liquid Nitrogen
Extensive facilities have been provided for repair, maintenance as well as
manufacture of spare parts. There is a structural shop, machine shop, founder, loco
repair shop, utility equipment repair ship. There is Electrical repair shop for repair
of electrical equipment.
1.4 MAJOR DEPARTMENTS
• Raw Material Handling Plant
• Coke ovens & Coal Chemical Plant (CO&CCP)
• Sinter Plant
• Blast Furnaces
• Steel Melt Shop
• Continuous casting Department
• Rolling Mills
• Light & Medium Merchant Mill (LMMM)
• Wire Rod Mill (WRM)
• Medium Merchant & Structural Mill (MMSM)
1.5 AUXILIARY FACILITIES
• Thermal Power Plant
• Distribution Network Department
• Traffic Department
• Engineering shops & Foundry (ES & F)
• Central Maintenance Electrical
• Electro Technical Laboratory
• Electrical Repair Shop (ERS)
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• Utilities Department
• Quality Assurance and Technological Development (QA &TD)
• Calcining & Refractory Material Plant
• Roll shop & Repair shop
• Plant Design
• Safety Engineering Department
• Spares Management Department
1.6 UTILITIES DEPARTMENT
Utilities department consists of
1. Air Separation Plant
2. Compressor Houses
3. Chilled water plants and
4. Acetylene plants.
The ASP is designed to meet the maximum daily demand of gaseous oxygen,
gaseous nitrogen and gaseous argon. Compressor Houses (CH) produce
Compressed Air required for the operation of pneumatic devices, for instruments
and controls, pneumatic tools and for general purpose in the various production units
of Steel Plants. Chilled Water plants (2 Nos.) produce chilled water required for use
in the ventilation and air conditioning system in areas such as office rooms,
electrical control room etc. Acetylene plant produces Acetylene gas required for
general purpose cutting and welding.
2.0 AIR SEPARATION PLANT
Air separation plant is one of the major auxiliary units and is designed to meet the
maximum daily demand of gaseous oxygen, gaseous nitrogen and gaseous argon.
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The plant has provision for production of liquid oxygen and liquid nitrogen for
storage and utilization during period of shutdown of the plant.
2.1 MAJOR CONSUMERS:
Total requirement of oxygen, nitrogen and argon all over the plant for stage is
24,248 Nm3/hr., 58,500 Nm3/Hr. and 32,000 Nm3/hr. respectively. Out of this SMS
requires 97.3% of O2 for LD Converters and LD vessels meeting, Blast Furnace
requires 63.4% of Nitrogen produced, Argon for homogenization of steel is 93.75%.
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2.2 BRIEF PROCESS:
Air is sucked from the atmosphere through a pulse type filter where dust is removed
and then compressed in a compressor to 7.4 KSCA. The air is pre cooled in air water
tower to 100C and sent to purification unit for the removal of moisture, CO2 and
hydrocarbons.
The purified air passes through the exchanger where it is cooled to its dew point
counter currently with the outgoing product i.e., oxygen, N2 and waste N2 from the
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distillation column. A part of air is taken at an intermediate point and expanded in
an expansion turbine to provide necessary cold to compensate the thermal losses of
the system. The air from exchanges will be sent to distillation system which will
separate air into oxygen, nitrogen and argon.
For the production of argon, a gaseous flow is picked up at a suitable point in the
upper column of the distillation system where the argon content is maximum
possible and sent to crude argon unit to produce crude argon containing 2-3%
oxygen and small amount of nitrogen as impurities. Oxygen is separated in warm
argon unit where oxygen is reacted with hydrogen in presence of palladium catalyst.
Hydrogen required will be taken from water electrolysis plant Nitrogen is separated
by distillation in pure argon column.
2.3 STORAGE & DISTRIBUTION:
Gaseous oxygen, nitrogen from the cold box is compressed to 40 KSCA to 10 KSCA
respectively and supplied directly to the consumer through pipe lines. The liquid
oxygen, nitrogen will be stored in liquid storage tanks and pumped to 40 KSCA by
centrifugal pumps and vaporized by steam water baths and supplied to consumer. In
case of emergency, liquid argon from cold box is collected in liquid argon tanks /
cold converters pressurized to 7 KSCA by tank values, vaporized and delivered to
consisting department normally.
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2.4 CYLINDER FILLING STATION:
Liquid oxygen, nitrogen and argon will be pumped by reciprocating pump to a
pressure of 150 KSCA, vaporized and filled to cylinders through manifolds.
3.0 DESCRIPTION OF ASU I, II AND III
The compressed air, delivered by one turbo-compressor C1, C2, C3 or C4, at a
temperature of 45ᴼC, is cooled down to 10ᴼC in the tower E10.
The air water tower E10 uses water coming from the general circuit and cooled
down in the nitrogen tower E11 by means of refrigeration contained in waste
nitrogen coming from the cold box. A refrigeration unit X01 on this water circuit
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brings additional refrigeration make-up, so that the temperature of the water is
brought down to 5ᴼC before the water enters the Air-water tower E10.
At the outlet of this cooling system, the air passes through a drying and carbon-
dioxide removal unit, composed of two bottles (R01 and R02), filled with alumina
in the bottom and molecular sieves at the top, the air flow going from bottom to top.
Moisture is adsorbed by alumina and CO2 by molecular sieves. One of the two
vessels is in operation while the other is reactivated by waste nitrogen coming from
the cold box. The total cycle is 10 hours (5 hours of adsorption and 5 hours of
regeneration) with 10ᴼC at the inlet of the bottles.
Then the compressed air, dry and CO2 free, is filtered to eliminate dust particles it
may carry by post-filters F11or F12, and then enters the cold box. It is cooled by
exchangers in counter current with the gaseous products of the separation (Oxygen
and Nitrogen) in the main exchangers (E01 and E02). A part of the air is taken at an
intermediate point of the exchangers and sent to a centrifugal expansion turbine
(D01 and D02) equipped with a brake generator to provide the necessary
refrigeration to make-up for the thermal losses of the cold box (the energy recovered
in the brake generator is sent to the high voltage (6.6kV) network of the plant)
The adiabatic expansion in D01/D02 to approximately atmospheric pressure
provides the main part of refrigeration required by the plant.
The other part is cooled in E01/E02 and sent to the bottom of the medium pressure
column K01 to perform the first separation.
In the column K01, the up flowing gas becomes enriched with nitrogen by contact
with the down coming liquid. This liquid result from the nitrogen condensation in
the vaporiser/condenser E03 located at the top of the column.
From the top to bottom, the medium pressure column K01 gives the following
products:
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1. Pure liquid nitrogen.
2. Poor liquid with low oxygen content, used as reflex on the top of the low
pressure column after passing through sub-cooler E05 and expansion in the
valve.
3. Rich liquid at the bottom, a liquid containing about 40% oxygen sent to the
low pressure column K02, after passing through E04 and expansion in the
valve.
The up flowing gas in the low pressure column K02 is the gas vaporised in the
vaporiser/condenser E03 located at the bottom of this column. This exchanger E03
is called main vaporiser, and vaporises liquid oxygen at low pressure by
condensation of medium pressure nitrogen in K01. The rich liquid sub-cooler E04
makes it possible for the liquid oxygen to flow though filter (R03 or R04) by a
thermo-siphon effect. This extra filtration permits to avoid abnormal accumulation
of dangerous products in the oxygen bath in case of accidental passage of impurities.
The low pressure column K02 gives:
1. At the bottom, liquid oxygen at a purity of 99.5%. Part of this liquid oxygen
is sent directly at the outlet of the cold box of storage. The other part is
vaporised in the air liquefier E06, and then heated in the main exchanger line
E02-E01.
2. At the top waste nitrogen which is heated in the main exchangers line E02-
E01.
The pure nitrogen column K03 is located above the low pressure column K02 and
gives at its top pure gaseous nitrogen at 99.9% purity, which is also heated in the
main heat exchanger E02-E02.
For recovery of Argon, a gaseous flow is picked up from the low pressure column
K02 at a point where Argon content is about the maximum possible. This fraction is
taken to the bottom of the crude argon columnK10. This flow rises to the top of the
column where it is partially condensed by vaporisation of “rich liquid” in the
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condenser/vaporiser E40. The liquefied fraction flows back down to the low
pressure column K02. The gaseous fraction called “crude argon” contains normally
2-3% oxygen and a small amount of nitrogen. Oxygen is eliminated in an external
warm section and nitrogen is eliminated at low temperature.
The crude argon is warmed to ambient temperature in exchanger E41 by exchange
with oxygen purified mixture. Then, the crude argon is compressed up to about
5kg/cm2 after injection of the right amount of hydrogen. Then it passes through the
catalytic reactor r10 filled with special catalyst where hydrogen reacts with the
oxygen contained in the mixture to form water.
At the outlet of the catalytic reactor R10, the gas contains less than 2 ppm of oxygen
but contains water as moisture, and its temperature is about 400ᴼC. By atmospheric
cooler (E42), water cooler (E43) and refrigeration unit (X10), the temperature of the
gas decreases to about 5ᴼC before entering the water separator (B11), where
condensed water is drained. Then, oxygen purified mixture is dried in a drying bottle
(R11 or R12) while the other is reactivated by dry heated argon.
The fraction used for reactivation is sent back, after expansion to the suction of the
crude argon (C40).
The dry fraction leaving the drying bottle (R11 or R12) is cooled down in exchanger
(E41) to its dew point and liquefied in condenser (E45) at medium pressure. After
being expanded through a valve, the liquid is sent to column (F11), and the gaseous
fraction produced by expansion containing the main part of excess hydrogen is
warmed in exchanger (E41) and sent to the suction of crude argon compressor.
The column (K11) is the final element for argon purification. It is fed at the mid part
by liquid argon mixture containing nitrogen and a small amount of hydrogen. This
column operates between a condenser fed by poor liquid, and the vaporiser (E45).
The pure liquid argon boiling at the bottom of the column (K11) is drawn off as pure
liquid argon product.
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3.1 MAJOR PARTS OF ASU
3.1.1 COLD BOX
In order to reduce the cold losses of the unit, all equipment operating at low
temperature as well as the connecting piping and valves are placed in a so called
“cold box”, and an insulating material (slag wool) is packed in the free space
between vessels, piping and cold box.
The cold box consists of steel panelled structure as tight as possible, in which a
positive pressure of dry nitrogen is maintained to prevent the entry of atmospheric
air/moisture.
The main box, containing the columns, has the following dimensions:
4800mm X 5600mm, Height = 28800mm
The exchangers are kept in another cold box near the main box. Its dimensions are:
5300mm X 6300mm, Height = 9650mm
Filters and vaporiser (E06) are placed in two separate boxes in which filter is kept
above the vaporiser. Each box has the following dimensions:
3500mm X 5600mm, Height = 9650mm
A separate box is used for cold argon equipment which is used for purification of
argon. They are placed above the heat exchangers. Their dimensions are:
2050mm X 2700mm, Height = 13400mm
3.1.2 MEDIUM PRESSURE COLUMN (K01)
Diameter = 2600mm
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Height = 9180mm
Weight empty = 11400kg
Weight operation = 18700kg
Trays = 46
3.1.3 LOW PRESSURE COLUMN (K02)
Diameter = 3600mm
Height = 10780mm
Weight empty = 15065kg
Weight operation = 28500kg
Trays = 72
3.1.4 PURE NITROGEN COLUMN (K03)
Diameter = 2400mm
Height = 3680mm
Weight empty = 2500kg
Weight operation = 3700kg
Trays = 17
3.1.5 MAIN VAPORISER (E03)
It is present between medium pressure column and low pressure column. This
vaporiser condenses the nitrogen on medium pressure column side and vaporises the
liquid oxygen on low pressure column side. It is composed of 4 cores in parallel.
Diameter = 3600mm
Height = 4200mm
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Weight empty = 12100kg
Weight operation = 35000kg
3.1.6 AIR LIQUEFIER (E06)
A part of air coming from the exchangers (E01-E02) is liquefied in counter current
of liquid oxygen coming from the main vaporiser (E03). The vaporised oxygen is
sent to the exchangers and the excess of liquid oxygen sent to the liquid oxygen
pumps and then to the storage. It is composed of one core.
1140mm X 4700mm, Height = 1850mm and Weight = 4200kg
3.2 ASP PRODUCTS AND THEIR APPLICATIONS
Air composition under atmospheric pressure: 760 mmHg (atmospheric pressure at
sea level) and at ambient temperature. The steam content in volume is variable
because it depends on the relative moisture. The content in carbon dioxide is
variable but is often lower or equal to 480 ppm (maximum current content, even in
the industrial field), that is why this value is, unless further information, the basis
for the sizing of front end purifications.
Gaseous
oxygen
SMS Converter blowing
BF Cold blast enrichment
SMS Slag splashing
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Gaseous
nitrogen
CDCP Coke dry quenching
BF BF top gear box cooling
Liquid
nitrogen,
Liquid oxygen,
Liquid argon
Other agencies Sales
Back-up system For gaseous pipe line network as an
emergency back up support
Gaseous argon SMS Argon rinsing
Oxygen,
Nitrogen and
argon cylinders
Various shops Welding or purging purposes
3.3 MASS AND THERMAL BALANCE
3.3.1 AIR FROM AC1, AC2, AC3, AC4
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 63900 7.0 45 59600 7.0 45
80% 77800 7.0 45 72000 7.0 45
100% 91200 7.0 45 84900 7.0 45
110% 100800 7.0 45 93400 7.0 45
3.3.2 WASTE NITROGEN TO ADSORPTION TOWER
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
18
60% 16000 1.2 13 15700 1.2 12
80% 16700 1.2 13 16400 1.2 12
100% 17450 1.2 13 17100 1.2 12
110% 18000 1.2 13 17550 1.2 12
3.3.3 WASTE NITROGEN TO E11
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 23458.7 1.2 13 16893.7 1.2 12
80% 23612.4 1.2 13 19692.4 1.2 12
100% 33216.1 1.2 13 22991.1 1.2 12
110% 33243.4 1.2 13 26590.4 1.2 12
3.3.4 AIR TO EXPANSION TURBINE
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 22400 6.5 -135 12322 6.61 -135
80% 26000 6.5 -135 12980 6.68 -135
100% 30000 6.5 -135 13640 6.75 -135
110% 32847 6.5 -135 13971 6.68 -135
3.3.5 GASEOUS OXYGEN TO COMPRESSORS
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
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60% 7650 1.7 13 8880 1.7 12
80% 10200 1.7 13 11840 1.7 12
100% 12750 1.7 13 14800 1.7 12
110% 14025 1.7 13 16280 1.7 12
3.3.6 GASEOUS NITROGEN TO COMPRESSORS
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 15300 1.2 13 17760 1.2 12
80% 20400 1.2 13 23680 1.2 12
100% 25500 1.2 13 29600 1.2 12
110% 28050 1.2 13 32560 1.2 12
3.3.7 LIQUID OXYGEN TO STORAGE
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 635 1.9 -185 110 1.9 -185
80% 810 1.9 -185 110 1.9 -185
100% 985 1.9 -185 110 1.9 -185
110% 1072 1.9 -185 110 1.9 -185
3.3.8 LIQUID NITROGEN TO STORAGE
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
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60% 787 1.5 -192.2 187 1.5 -192.2
80% 987 1.5 -192.2 187 1.5 -192.2
100% 1187 1.5 -192.2 187 1.5 -192.2
110% 1287 1.5 -192.2 187 1.5 -192.2
3.3.9 LIQUID ARGON TO STORAGE
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 65 1.05 -185.6 65 1.05 -185.6
80% 85 1.05 -185.6 85 1.05 -185.6
100% 105 1.05 -185.6 105 1.05 -185.6
110% 115 1.05 -185.6 115 1.05 -185.6
3.3.10 WASTE GASES
MIXED GASEOUS
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
FLOW
Nm3/h
PRESSURE
KSCA
TEMP.
ᴼC
60% 4.3 1.05 -189 4.3 1.05 -185.6
80% 5.6 1.05 -189 5.6 1.05 -185.6
100% 6.9 1.05 -189 6.9 1.05 -185.6
110% 7.6 1.05 -189 7.6 1.05 -185.6
4.0 DISTILLATION
Distillation is a method of separating mixtures based on differences in volatilities of
components in a boiling liquid mixture. Distillation is a unit operation, or a physical
separation process, and not a chemical reaction.
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Commercially, distillation has a number of applications. It is used to separate crude
oil into more fractions for specific uses such as transport, power generation and
heating. Water is distilled to remove impurities, such as salt from seawater. Air is
distilled to separate its components—notably oxygen, nitrogen, and argon— for
industrial use. Distillation of fermented solutions has been used since ancient times
to produce distilled beverages with a higher alcohol content. The premises where
distillation is carried out, especially distillation of alcohol, are known as a distillery.
4.1 HISTORY
The first clear evidence of distillation comes from Greek alchemists working in
Alexandria in the first century AD. Distilled water has been known since at least ca.
200 AD, when Alexander of Aphrodisias described the process. Arabs learned the
process from the Egyptians and used it extensively in their chemical experiments.
Clear evidence of the distillation of alcohol comes from the School of Salerno in the
12th century. Fractional distillation was developed by Tadeo Alderotti in the 13th
century.
In 1500, German alchemist Hieronymus Braunschweig published Liber de arte
destillandi (The Book of the Art of Distillation) the first book solely dedicated to
the subject of distillation, followed in 1512 by a much expanded version. In 1651,
John French published The Art of Distillation the first major English compendium
of practice, though it has been claimed that much of it derives from Braunschweig's
work. This includes diagrams with people in them showing the industrial rather than
bench scale of the operation.
As alchemy evolved into the science of chemistry, vessels called retorts became
used for distillations. Both alembics and retorts are forms of glassware with long
necks pointing to the side at a downward angle which acted as air-cooled condensers
to condense the distillate and let it drip downward for collection. Later, copper
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alembics were invented. Riveted joints were often kept tight by using various
mixtures, for instance dough made of rye flour. These alembics often featured a
cooling system around the beak, using cold water for instance, which made the
condensation of alcohol more efficient. These were called pot stills. Today, the
retorts and pot stills have been largely supplanted by more efficient distillation
methods in most industrial processes. However, the pot still is still widely used for
the elaboration of some fine alcohols such as cognac, Scotch whisky, tequila and
some vodka. Pot stills made of various materials (wood, clay, stainless steel) are
also used by bootleggers in various countries. Small pot stills are also sold for the
domestic production of flower water or essential oils.
Early forms of distillation were batch processes using one vaporization and one
condensation. Purity was improved by further distillation of the condensate. Greater
volumes were processed by simply repeating the distillation. Chemists were
reported to carry out as many as 500 to 600 distillations in order to obtain a pure
compound.
In the early 19th century the basics of modern techniques including pre-heating and
reflux were developed, particularly by the French, then in 1830 a British Patent was
issued to Aeneas Coffey for a whiskey distillation column, which worked
continuously and may be regarded as the archetype of modern petrochemical units.
In 1877, Ernest Solvay was granted a U.S. Patent for a tray column for ammonia
distillation and the same and subsequent years saw developments of this theme for
oil and spirits.
With the emergence of chemical engineering as a discipline at the end of the 19th
century, scientific rather than empirical methods could be applied. The developing
petroleum industry in the early 20th century provided the impetus for the
development of accurate design methods such as the McCabe-Thiele method and
the Fenske equation. The availability of powerful computers has also allowed direct
computer simulation of distillation columns.
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4.2 TYPES OF DISTILLATION
4.2.1 SIMPLE DISTILLATION
In simple distillation, all the hot vapors produced are immediately channelled into a
condenser that cools and condenses the vapors. Therefore, the distillate will not be
pure - its composition will be identical to the composition of the vapors at the given
temperature and pressure, and can be computed from Raoult's law.
As a result, simple distillation is usually used only to separate liquids whose boiling
points differ greatly (rule of thumb is 25 °C), or to separate liquids from in-volatile
solids or oils. For these cases, the vapor pressures of the components are usually
sufficiently different that Raoult's law may be neglected due to the insignificant
contribution of the less volatile component. In this case, the distillate may be
sufficiently pure for its intended purpose.
4.2.2 FRACTIONAL DISTILLATION
For many cases, the boiling points of the components in the mixture will be
sufficiently close that Raoult's law must be taken into consideration. Therefore,
fractional distillation must be used in order to separate the components well by
repeated vaporization-condensation cycles within a packed fractionating column.
This separation, by successive distillations, is also referred to as rectification.
As the solution to be purified is heated, its vapors rise to the fractionating column.
As it rises, it cools, condensing on the condenser walls and the surfaces of the
packing material. Here, the condensate continues to be heated by the rising hot
vapors; it vaporizes once more. However, the composition of the fresh vapors is
determined once again by Raoult's law. Each vaporization-condensation cycle
(called a theoretical plate) will yield a purer solution of the more volatile component.
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In reality, each cycle at a given temperature does not occur at exactly the same
position in the fractionating column; theoretical plate is thus a concept rather than
an accurate description.
More theoretical plates lead to better separations. A spinning band distillation
system uses a spinning band of Teflon or metal to force the rising vapors into close
contact with the descending condensate, increasing the number of theoretical plates.
4.2.3 STEAM DISTILLATION
Like vacuum distillation, steam distillation is a method for distilling compounds
which are heat-sensitive. The temperature of the steam is easier to control than the
surface of a heating element, and allows a high rate of heat transfer without heating
at a very high temperature. This process involves bubbling steam through a heated
mixture of the raw material. By Raoult's law, some of the target compound will
vaporize (in accordance with its partial pressure). The vapor mixture is cooled and
condensed, usually yielding a layer of oil and a layer of water.
Steam distillation of various aromatic herbs and flowers can result in two products;
an essential oil as well as a watery herbal distillate. The essential oils are often used
in perfumery and aromatherapy while the watery distillates have many applications
in aromatherapy, food processing and skin care.
4.2.4 VACUUM DISTILLATION
Some compounds have very high boiling points. To boil such compounds, it is often
better to lower the pressure at which such compounds are boiled instead of
increasing the temperature. Once the pressure is lowered to the vapor pressure of
the compound (at the given temperature), boiling and the rest of the distillation
25
process can commence. This technique is referred to as vacuum distillation and it is
commonly found in the laboratory in the form of the rotary evaporator.
This technique is also very useful for compounds which boil beyond their
decomposition temperature at atmospheric pressure and which would therefore be
decomposed by any attempt to boil them under atmospheric pressure.
Molecular distillation is vacuum distillation below the pressure of 0.01 torr is one
order of magnitude above high vacuum, where fluids are in the free molecular flow
regime, i.e. the mean free path of molecules is comparable to the size of the
equipment. The gaseous phase no longer exerts significant pressure on the substance
to be evaporated, and consequently, rate of evaporation no longer depends on
pressure. That is, because the continuum assumptions of fluid dynamics no longer
apply, mass transport is governed by molecular dynamics rather than fluid
dynamics. Thus, a short path between the hot surface and the cold surface is
necessary, typically by suspending a hot plate covered with a film of feed next to a
cold plate with a line of sight in between. Molecular distillation is used industrially
for purification of oils.
4.2.5 AIR-SENSITIVE VACUUM DISTILLATION
Some compounds have high boiling points as well as being air sensitive. A simple
vacuum distillation system as exemplified above can be used, whereby the vacuum
is replaced with an inert gas after the distillation is complete. However, this is a less
satisfactory system if one desires to collect fractions under a reduced pressure. To
do this a "cow" or "pig" adaptor can be added to the end of the condenser, or for
better results or for very air sensitive compounds a Perkin triangle apparatus can be
used.
The Perkin triangle, has means via a series of glass or Teflon taps to allows fractions
to be isolated from the rest of the still, without the main body of the distillation
26
being removed from either the vacuum or heat source, and thus can remain in a state
of reflux. To do this, the sample is first isolated from the vacuum by means of the
taps; the vacuum over the sample is then replaced with an inert gas (such as nitrogen
or argon) and can then be stoppered and removed. A fresh collection vessel can then
be added to the system, evacuated and linked back into the distillation system via
the taps to collect a second fraction, and so on, until all fractions have been collected.
4.2.6 SHORT PATH DISTILLATION
Short path distillation is a distillation technique that involves the distillate travelling
a short distance, often only a few centimetres, and is normally done at reduced
pressure.[19] A classic example would be a distillation involving the distillate
travelling from one glass bulb to another, without the need for a condenser
separating the two chambers. This technique is often used for compounds which are
unstable at high temperatures or to purify small amounts of compound. The
advantage is that the heating temperature can be considerably lower (at reduced
pressure) than the boiling point of the liquid at standard pressure, and the distillate
only has to travel a short distance before condensing. A short path ensures that little
compound is lost on the sides of the apparatus. The Kugelrohr is a kind of a short
path distillation apparatus which often contain multiple chambers to collect distillate
fractions.
4.2.7 ZONE DISTILLATION
Zone distillation is a distillation process in long container with partial melting of
refined matter in moving liquid zone and condensation of vapor in the solid phase
at condensate pulling in cold area. The process is worked in theory. When zone
heater is moving from the top to the bottom of the container then solid condensate
with irregular impurity distribution is forming. Then most pure part of the
27
condensate may be extracted as product. The process may be iterated many times
by moving (without turnover) the received condensate to the bottom part of the
container on the place of refined matter. The irregular impurity distribution in the
condensate (that is efficiency of purification) increases with number of repetitions
of the process. Zone distillation is a distillation analog of zone recrystallization.
Impurity distribution in the condensate is described by known equations of zone
recrystallization with various numbers of iteration of process – with replacement
distribution efficient k of crystallization on separation factor α of distillation.
4.3 INDUSTRIAL DISTILLATION
Large scale industrial distillation applications include both batch and continuous
fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely
used industrial applications of continuous, steady-state fractional distillation are in
28
petroleum refineries, petrochemical and chemical plants and natural gas processing
plants.
Industrial distillation is typically performed in large, vertical cylindrical columns
known as distillation towers or distillation columns with diameters ranging from
about 65 centimetres to 16 meters and heights ranging from about 6 meters to 90
meters or more. When the process feed has a diverse composition, as in distilling
crude oil, liquid outlets at intervals up the column allow for the withdrawal of
different fractions or products having different boiling points or boiling ranges. The
"lightest" products (those with the lowest boiling point) exit from the top of the
columns and the "heaviest" products (those with the highest boiling point) exit from
the bottom of the column and are often called the bottoms.
Industrial towers use reflux to achieve a more complete separation of products.
Reflux refers to the portion of the condensed overhead liquid product from a
distillation or fractionation tower that is returned to the upper part of the tower as
shown in the schematic diagram of a typical, large-scale industrial distillation tower.
Inside the tower, the down flowing reflux liquid provides cooling and condensation
of the up flowing vapors thereby increasing the efficiency of the distillation tower.
The more reflux provided for a given number of theoretical plates, the better the
tower's separation of lower boiling materials from higher boiling materials.
Alternatively, the more reflux that is provided for a given desired separation, the
fewer the number of theoretical plates required.
Such industrial fractionating towers are also used in air separation, producing liquid
oxygen, liquid nitrogen, and high purity argon. Distillation of chloro-silanes also
enables the production of high-purity silicon for use as a semiconductor.
Design and operation of a distillation tower depends on the feed and desired
products. Given a simple, binary component feed, analytical methods such as the
McCabe-Thiele method or the Fenske equation can be used. For a multi-component
feed, simulation models are used both for design and operation. Moreover, the
29
efficiencies of the vapor-liquid contact devices (referred to as "plates" or "trays")
used in distillation towers are typically lower than that of a theoretical 100%
efficient equilibrium stage. Hence, a distillation tower needs more trays than the
number of theoretical vapor-liquid equilibrium stages.
In modern industrial uses, a packing material is used in the column instead of trays
when low pressure drops across the column are required. Other factors that favour
packing are: vacuum systems, smaller diameter columns, corrosive systems,
systems prone to foaming, systems requiring low liquid holdup and batch
distillation. Conversely, factors that favour plate columns are: presence of solids in
feed, high liquid rates, large column diameters, complex columns, columns with
wide feed composition variation, columns with a chemical reaction, absorption
columns, columns limited by foundation weight tolerance, low liquid rate, large
turn-down ratio and those processes subject to process surges.
This packing material can either be random dumped packing (1-3" wide) such as
Raschig rings or structured sheet metal. Liquids tend to wet the surface of the
packing and the vapors pass across this wetted surface, where mass transfer takes
place. Unlike conventional tray distillation in which every tray represents a separate
point of vapor-liquid equilibrium, the vapor-liquid equilibrium curve in a packed
column is continuous. However, when modelling packed columns, it is useful to
compute a number of "theoretical stages" to denote the separation efficiency of the
packed column with respect to more traditional trays. Differently shaped packings
have different surface areas and void space between packings. Both of these factors
affect packing performance.
Another factor in addition to the packing shape and surface area that affects the
performance of random or structured packing is the liquid and vapor distribution
entering the packed bed. The number of theoretical stages required to make a given
separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor
are not evenly distributed across the superficial tower area as it enters the packed
bed, the liquid to vapor ratio will not be correct in the packed bed and the required
30
separation will not be achieved. The packing will appear to not be working properly.
The height equivalent of a theoretical plate (HETP) will be greater than expected.
The problem is not the packing itself but the mal-distribution of the fluids entering
the packed bed. Liquid mal-distribution is more frequently the problem than vapor.
The design of the liquid distributors used to introduce the feed and reflux to a packed
bed is critical to making the packing perform to it maximum efficiency. Methods of
evaluating the effectiveness of a liquid distributor to evenly distribute the liquid
entering a packed bed can be found in references. Considerable work has been done
on this topic by Fractionation Research, Inc. (commonly known as FRI).
4.3.1 MULTI-EFFECT DISTILLATION
The goal of multi-effect distillation is to increase the energy efficiency of the
process, for use in desalination, or in some cases one stage in the production of
ultrapure water. The number of effects is proportional to the kW·h/m3 of water
recovered figure, and refers to the volume of water recovered per unit of energy
compared with single-effect distillation. One effect is roughly 636 kW·h/m3.
Multi-stage flash distillation can achieve more than 20 effects with thermal energy
input, as mentioned in the article.
Vapor compression evaporation Commercial large-scale units can achieve around
72 effects with electrical energy input, according to manufacturers.
There are many other types of multi-effect distillation processes, including one
referred to as simply multi-effect distillation (MED), in which multiple chambers,
with intervening heat exchangers, are employed.
5.0 CRYOGENIC DISTILLATION
There are theoretically three means of obtaining oxygen:
31
Electrical : water electrolysis
Mechanical : air centrifuging
Chemical : solubility in various liquids, separation by passing
through porous materials.
But, the only procedure widely used on an industrial scale, consists in extracting
oxygen and other components of air from air by liquefaction and distillation at
cryogenic temperatures.
An air separation plant separates atmospheric air into its primary components,
typically nitrogen and oxygen sometimes also argon and other rare inert gases. There
are various technologies that are used for the separation process; the most common
is via cryogenic distillation. This process was pioneered by Dr. Carl von Linde in
the early 20th century and is still used today to produce high purity gases. The
cryogenic separation process requires a very tight integration of heat exchangers and
separation columns to obtain a good efficiency and all the energy for refrigeration
is provided by the compression of the air at the inlet of the unit. In addition to the
cryogenic distillation method there are other methods such as Membrane, Pressure
Swing Adsorption (PSA) and Vacuum Pressure Swing Adsorption (VPSA), which
are typically used to separate a single component from ordinary air. Production of
high purity oxygen, nitrogen, and argon as used for Semiconductor device
fabrication requires cryogenic distillation, though. Similarly, the only viable sources
of the rare gases neon, krypton, and xenon are the distillation of air using at least
two distillation columns. Cryogenic ASU's are built to provide nitrogen and/or
oxygen and often co-produce argon where liquid products (Liquid nitrogen "LIN",
Liquid oxygen "LOX", and Liquid argon "LAR") can only be produced if sufficient
refrigeration is provided for in the design.
5.1 COMPOSITION OF AIR
32
5.2 CRYOGENIC PROCESS
To achieve the low distillation temperatures an Air Separation Unit (ASU) requires
a refrigeration cycle that operates by means of the Joule–Thomson effect, and the
cold equipment has to be kept within an insulated enclosure (commonly called a
"cold box"). The cooling of the gases requires a large amount of energy to make this
refrigeration cycle work and is delivered by an air compressor. Modern ASU's use
33
Turbo-expanders for cooling combined with the air compressor for improved
efficiency. The process consists of the following main steps:
1. Before compression the air is pre-filtered of dust.
2. Air is compressed where the final delivery pressure is determined by
recoveries and the fluid state (gas or liquid) of the products. Typical pressures
range between 5 and 10 bar gauge. The air stream may also be compressed to
different pressures to enhance the efficiency of the ASU. During compression
water is condensed out in inter-stage coolers.
3. The process air is generally passed through a molecular sieve bed, which
removes any remaining water vapour, as well as carbon dioxide, which would
freeze and plug the cryogenic equipment. Molecular sieves are often designed
to remove any gaseous hydrocarbons from the air, since these can be a
problem in the subsequent air distillation that could lead to explosions. The
molecular sieves bed must be regenerated. This is done by installing multiple
units operating in alternating mode and using the dry co-produced waste gas
to desorb the water.
4. Process air is passed through an integrated heat exchanger (usually a plate fin
heat exchanger) and cooled against product (and waste) cryogenic streams.
Part of the air liquefies to form a liquid that is enriched in oxygen. The
remaining gas is richer in nitrogen and is distilled to almost pure nitrogen
(typically < 1ppm) in a high pressure (HP) distillation column. The condenser
of this column requires refrigeration which is obtained from expanding the
more oxygen rich stream further across a valve or through an Expander, (a
reverse compressor).
5. Alternatively the condenser may be cooled by interchanging heat with a
reboiler in a low pressure (LP) distillation column (operating at 1.2-1.3 bar
abs.) when the ASU is producing pure oxygen. To minimize the compression
cost the combined condenser/reboiler of the HP/LP columns must operate
with a temperature difference of only 1-2 degrees Kelvin, requiring plate fin
34
brazed aluminium heat exchangers. Typical oxygen purities range in from
97.5% to 99.5% and influence the maximum recovery of oxygen. The
refrigeration required for producing liquid products is obtained using the JT
effect in an expander which feeds compressed air directly to the low pressure
column. Hence, a certain part of the air is not to be separated and must leave
the low pressure column as a waste stream from its upper section.
6. Because the boiling point of argon (87.3 K at standard conditions) lies
between that of oxygen (90.2 K) and nitrogen (77.4 K), argon builds up in the
lower section of the low pressure column. When argon is produced, a vapor
side draw is taken from the low pressure column where the argon
concentration is highest. It is sent to another column rectifying the argon to
the desired purity from which liquid is returned to the same location in the
LP column. The uses of modern structured packings which have very low
pressure drops enable argon purities of less than 1 ppm. Though argon is
present in less to 1% of the incoming, the air argon column requires a
significant amount of energy due to the high reflux ratio required (about 30)
in the argon column. Cooling of the argon column can be supplied from cold
expanded rich liquid or by liquid nitrogen.
7. Finally the products produced in gas form are warmed against the incoming
air to ambient temperatures. This requires a carefully crafted heat integration
that must allow for robustness against disturbances (due to switch over of the
molecular sieve beds). It may also require additional external refrigeration
during start-up. The air gases are sometimes supplied by pipeline to large
industrial users adjacent to or nearby to the production plant. Unless a viable
pipeline system exists, long distance transportation of products is usually
done as a liquid product for large quantities or as dewar flasks or gas cylinders
for small quantities.
6.0 DEVELOPMANT OF DISTILLATION
35
In order to introduce the notion of distillation, let us take the well-known example
of water and alcohol separation. A distillation flask containing a water/alcohol liquid
mixture is available.
The basic principle of distillation is to separate a mixture in its more or less volatile
different components, that is to say components that do not vaporise at the same
temperature (under a given pressure). In this example, water vaporises, under
atmospheric pressure, at 100 °C, and alcohol at a lower temperature. Hence alcohol
is the most volatile component of the mixture.
6.1 IMPORTANT BASICS
6.1.1 PARTIAL PRESSURE
The partial pressure of a component is pressure under which this component would
be if it was alone to take up all of the mixture volume.
36
Example: let us consider a gas mixture under 1 bar made up of 21% oxygen and
79% nitrogen. If oxygen was alone to take up the same volume at the same
temperature, its pressure would be: 0.21 x 1 bar = 0.21 bar
This pressure is known as oxygen partial pressure in the considered mixture.
In the same mixture, the partial pressure of nitrogen is: 0.79 x 1 bar = 0.79 bar
6.1.2 CONCOMITANT PHASE
Given a liquid mixture made up of 21% oxygen and 79% nitrogen. Its temperature
is -183 °C. Vapour is in equilibrium with this liquid under the total pressure P. It is
known as concomitant phase of this liquid. How can the composition and pressure
of the vapour be determined?
In an approximate way, the composition of vapour can be calculated as follows: At
a given temperature, vapour tension is determined with the curves given in the
appendices. It is noticeable that vapour is richer in nitrogen than the liquid, which is
richer in oxygen than the vapour. At the equilibrium, the vapour phase is richer in
more volatile elements than the liquid phase, which is richer in less volatile elements
than the vapour phase. The phases of this mixture are therefore known as
concomitant for this equilibrium. All this characterises the basis phenomenon of
distillation.
On the other hand, one can notice that a liquid-vapour equilibrium state can be
characterised thanks to four parameters: pressure, temperature, liquid composition
and vapour composition, and that these four parameters depend on each other. In
fact, only two of them are required in order to determine the two others, and to
define completely the equilibrium state. That is why, if two parameters are known
for a mixture the phases of which are concomitant, the two others can be calculated
by the approximate way used above.
37
We would be better of using the concomitant phase’s diagram (enclosed in the
appendices at the end of this part) which gives, for a O2+ N2 mixture, the liquid and
vapour compositions under different pressures and at different temperatures.
6.1.3 RAOULT’S LAW
In a two-phase mixture in liquid-vapour equilibrium, partial pressure of a
component in vapour phase is equal to the product of vapour pressure with this
component content in the liquid.
6.1.4 EQUILIBRIUM COEFFICIENT OF A COMPONENT
K (most volatile component) > 1 and K (less volatile component) < 1
38
6.1.5 PHASES DIAGRAM UNDER CONSTANT PRESSURE
3 domains are determined on this diagram:
Domain where mixture is in liquid state
Domain where mixture is in a liquid + gaseous state (two-phase mixture),
Domain where mixture is in gaseous state.
The curve 1 which demarcates the L and L+V domains is known as boiling curve
(appearance of the first vapour bubble in the liquid) and the curve 2, which
demarcates the L+S and S domains, is known as dew curve (appearance of the first
liquid drop in the vapour).
One can note that in a mixture (unlike a pure component), the dew and boiling points
are not similar. For any point P ta ken in L+S domain, the curve 1 gives the liquid
composition x (point PL), and the curve 2 gives the vapour composition y (point PV).
If the phase diagrams of the same binary mixture are compared under two different
pressures, it can be noticed that, when the pressure increases, the diagram draws in
and goes up the temperature axis.
So we recognise the well-known effect of pressure on the fusion and boiling
temperatures, as well as on distillation, that is to say, the higher the pressure, the
smaller the difference of volatile component concentration between the liquid and
vapour phases, hence the less effective the distillation for a given number of trays.
39
6.1.5 PHASES DIAGRAM UNDER CONSTANT TEMPERATURE
Similarly to the phases diagram under constant pressure, the three domains L, L+V
and V, as well as the boiling and the dew curves in a constant temperature diagram
are determined. The change of state will be known as isotherm (at constant
temperature).
If the phase diagrams of the same binary mixture are compared at two different
temperatures, it can be noticed that, when the temperature increases, the diagram
draws in and goes up the pressure axis.
Thus the same conclusion can be reached as for the constant pressure diagram: the
higher the temperature, the smaller the difference of volatile component
concentration between the liquid phase and the vapour phase, hence the less
effective the distillation for a given number of trays.
6.2 PRINCIPLE OF FRACTIONAL DISTILLATION
40
Let us consider the example of the distillation flask. If the distillate obtained after
the first distillation is not pure enough, it is possible to repeat the operation. So the
liquid obtained after the first distillation can be treated in a new distillation flask in
order to increase its purity. Unfortunately, this first model requires as many boilers
and condensers as distillation flasks.
It can be seen in this new configuration that, for each distillation flask, the vapour
penetrates into the liquid: a liquid-vapour contact is realised (we can also speak
about bubbling). The rising vapour is warmer than the liquid in the distillation flask
it penetrates. So the vapour will be a bit cooled in each distillation flask. This
cooling will be accompanied by a partial condensation of the vapour. At the same
time the liquid in the distillation flask will be a bit heated up: this heating up will be
accompanied by a partial vaporisation of the liquid.
41
This piling-up of distillation flasks is quite satisfying from a theoretical point of
view but doubtlessly it is very difficult to realise in practice. That is why a third
piling-up model can be proposed with trays.
6.3 DISTILLATION TRAY
42
The aim of the distillation tray that can be associated with the distillation flask
mentioned before is to enable the contact of the descending liquid and the rising
vapour under the most favourable conditions. In fact the tray will be the theatre of
Cooling and partial condensation of the rising vapour.
Heating and partial vaporisation of the descending liquid
Ideally, these two exchanges of heat and matter will result in concomitance of
phases between the liquid and the vapour going out of the tray. Thus a tray’s
behaviour can be explained applying different laws in the section reminders of
thermodynamics to the liquid and the vapour leaving the tray, which are supposed
to be in equilibrium and therefore concomitant.
Yet it is easy to imagine that, no matter what technology is developed, it will be
very difficult to obtain perfect homogeneity on the tray and, therefore, concomitance
at any point. So we can speak about a real tray (as compared to the theoretical tray)
which can be defined by the following output, which defines the number of real
trays that must be fitted into a distillation column section according to the number
of necessary theoretical trays:
Tray output = [(number of theoretical trays) / (number of real trays)]
It can be noticed that the tray provides the rising gas with a certain resistance (the
tray is said to create a loss of load), and we will see that this loss of load must be as
small as possible. This parameter will certainly be of importance during the
development of a tray’s technology.
6.4 DISTILLATION PACKING
There exists a second technology, from which Air Liquide has developed its own
variant, which is called distillation packing. This concept differs from the distillation
tray as the distillation is no longer fractional (tray by tray) but is continuous.
43
Distillation packing is an assembly of corrugated iron sheets which, assembled
vertically, allow the gas to get into continuous vertical contact with a film of
descending liquid along the packing waves. As a result, we no longer speak about a
distillation tray (real or theoretical) but about a height equivalent to a theoretical
tray (HETP). The advantage of this technology is that, contrary to the distillation
tray, the gas rising in the packing “packs” has no longer to overcome the loss of load
equivalent to the liquid height on each tray as it is in constant contact with a film of
liquid. As a result a distillation column equipped with packing realises a much
smaller total loss of load than if it were equipped with trays. This saving is of great
economic importance since, as we can see further on, energy consumption depends
, among other things, on the loss of load created in the columns .
7.0 DISTILLATION DESIGN
44
The distillation column of ASU is kept in the cold box to maintain the low
temperature and high pressure. They use a double column model.
7.0.1 SINGLE COLUMN MODEL
FIRST CASE - REMOVAL OF THE CONDENSER:
First it can be noticed that if the mixture to be treated comes into the column in
vapour phase, it will no longer be condensed at the top of the column. In the column
there will be only one phase: the vapour phase. Therefore no distillation is possible,
as it requires both a vapour phase and a liquid phase (reflux liquid). So in the case
under consideration it is indispensable that the mixture to be treated should come
into the column in liquid phase. But then all trays situated above the entrance of the
mixture to be treated become useless as it is only the rising vapour which passes
through them (no reflux liquid runs through them against the vapour). They can be
removed and so this model can be obtained. This way at the bottom of the column
we obtain a liquid consisting of oxygen (the purity of which depends among other
things on the number of trays), and at the top of the column a vapour consisting of
nitrogen and oxygen: a residual product.
SECOND CASE - REMOVAL OF THE BOILER:
45
This time it can be noticed that if the mixture to be treated comes in in liquid phase,
no distillation is possible because of the absence of vapour. So in this case we will
be interested in vapour mixtures coming into the column. On the other hand, it is
only the descending reflux liquid which will run through the trays situated below
the entrance of the rising vapour mixture coming into the column, and no distillation
will occur there. So they can be removed and this model obtained. This way, in the
top of the column we obtain a vapour consisting of nitrogen (the purity of which
depends among other things on the number of trays), and in the bottom a liquid
consisting of O2 and N2 (a residual product).
This way from the boiler/condenser model we have created two models which have
the advantage of requiring only one of the two elements “boiler/condenser”, but
each one of which has the inconvenience of having only one exploitable product
(nitrogen or oxygen).
7.0.2 DOUBLE COLUMN MODEL
46
One can consider the idea that the two models schemed before could function
interdependently in the following way: The gaseous air would be injected at the
bottom of the top condenser column and would be separated in order to give
nitrogen. Part of this nitrogen would be condensed in order to ensure the presence
of reflux liquid in the column, this way making distillation possible. This
condensation would be realised through the exchange of heat with the liquid oxygen
which would be vaporised through the same heat exchange. This liquid oxygen
would be in the bottom of a second column, a bottom boiler one, whose feeding
with reflux liquid at the top would be ensured by the residual liquid in the first
column. This second column would have for waste a vapour mixture of nitrogen and
oxygen.
7.1 ASU I, II and III
47
48
We have already seen about the parts of the distillation column. They have three
columns Medium pressure, Low pressure and Nitrogen column. There are 46 trays
in MP, 72 trays in LP column and 15 in nitrogen column.
The feed air from the heat exchangers enters the MP column in the 10th tray. The
three products are pure nitrogen, poor liquid and rich liquid. Pure nitrogen is
collected from the 44th tray and sent to the nitrogen column for more purification
and separation of waste nitrogen. The rich liquid is sent as the reflex to the LP
column to the 66th tray. The poor liquid is taken from the 24th tray and sent as the
reflex to the LP column at the 72nd tray. Before changing the columns the products
pass through various heat exchangers and attain their required status
In the LP column the feed enters from the turbines at low pressures and enter the
60th tray. The top product, rich in nitrogen is sent to the nitrogen column as reflex.
Then lower plate product, rich is oxygen is taken as LOX storage. In the middle
level, in the 38th tray the argon content is maximum. This is taken to the argon
purification.
49
7.2 ASU IV
Designing a randomly packed column is a subtle blend of art and science. Packed
columns are most frequently used to remove contaminants from a gas stream
(absorption). However, packed columns can also be used to remove volatile
components from a liquid stream by contacting it with an inert gas (stripping). They
are also used in distillation applications where the separation is particularly difficult
due to close boiling components.
To meet the requirements of Oxygen, Nitrogen & Argon gases at 6.3 MTPA stage
an energy efficient 600 TPD oxygen production capacity Air Separation Plant i.e.,
ASP-4 is installed at the existing Air Separation unit area, supplied by M/s Air
Liquide Engineering India pvt. Ltd.
Some of the major features are:
• Structured packing of all columns which gives low pressure drop
• 15 % energy efficient i.e. 0.65 kWhr power is consumed for cum oxygen
production.
• Yield of oxygen is 20%
• Argon compression within cold box by pumping
• Booster Air compressor for 10% e liquid oxygen production.
• Oxygen compression within cold box by pumping liquid oxygen and
vaporisation
• Liquid argon storage facilities
• Cost of the project is Rs.14500 Lakhs.
This is also similar to the first three ASUs. The first three has tray towers and the
fourth one has structured packing. The old unit with sieve trays has a yield of only
16% oxygen but the new one gives an higher yield of 20%.
50
51
52
8.0 CONCLUSION
In most applications, the purpose of a packed bed is to provide intimate contacting
of the upward flowing vapor and the downward flowing liquid in separation
processes such as distillation and absorption.
In the packed bed, liquids tend to wet the surface of the packing and the vapors pass
across this wetted surface, where mass transfer takes place. Packing material can be
used instead of trays or plates to improve separation in distillation columns. Packing
offers the advantage of a lower pressure drop across the column when compared to
trays or plates, which is especially beneficial when used in vacuum distillation
columns.
Differently shaped packing materials have different surface areas and different
amounts of void space. Both of these factors affect packing performance. In general,
the more surface area for a given volume of packing material, the better is the
performance of the packing.
Another factor affecting performance, in addition to packing shape and surface area,
is the distribution of vapor and liquid as they that enters the packed bed. The number
of theoretical stages required to make a given separation is calculated is a function
of the vapor to liquid ratio. If the liquid and vapor are not evenly distributed across
the packed bed, the desired separation will not be achieved. The problem is not the
packing itself but the mal-distribution of the fluids entering the packed bed. As
shown in Figure 3, columns containing packed beds are designed to include liquid
distributors so as to distribute the liquid evenly over the cross-sectional area of the
packing in order to optimize the efficiency of the mass transfer. Methods of
evaluating the effectiveness of a liquid distributor can be found in the technical
literature.
Packed columns have a continuous vapor-liquid equilibrium curve, unlike
conventional tray distillation in which every tray represents a separate point of
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vapor-liquid equilibrium. However, when designing packed columns it is useful to
first determine the number of theoretical equilibrium stages required for the desired
separation. Then the packing height needed to constitute a theoretical equilibrium
stage, known as the height equivalent to a theoretical plate (HETP), is also
determined. The total packing height required is the number theoretical equilibrium
stages multiplied by the HETP.