fundamentals of distillation
TRANSCRIPT
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A Sunrise Project for a Sunshine Future
PARADEEP REFINERY PROJECT
FacultyM Elamaran
PNM-CG
Prepared byProduction-CG Team
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CONTENTS
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Introduction
Basics of distillation
History of distillation
Types of distillation
Videos
Fractionating equipments
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INTRODUCTION TO REFINERY
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Hydrocarbon molecules in crude do NOT meet customer needsSEPARATION PROCESSESPrimary Processes)
CONVERSION PROCESSESSecondary Processes)FINISHING PROCESSESSecondary Processes)
Segregate the molecules
Rearrange the molecules
Remove Contaminants
MARKETABLE PRODUCTS
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UNIT OPERATIONS
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1.Fluid flow processes, including fluids transportation, filtration, and solidsfluidization.
2. Heat transfer processes, including evaporation, condensation, and heatexchange.
3. Mass transfer processes, including gas absorption, distillation, extraction,
adsorption, and drying.
4.Thermodynamic processes, including gas liquefaction, and refrigeration.
5.Mechanical processes, including solids transportation, crushing and
pulverization, and screening and sieving.
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UNIT OPERATIONS
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1. Combination.
2. SeparationDistillation comes under this.
3. Reaction.
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BASICS OF DISTILLATION
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EVAPORATION
ORDINARY EVAPORATION IS A SURFACE PHENOMENON- SOME MOLECULESHAVE ENOUGH KINETIC ENERGY TO ESCAPE FROM THE LIQUID.
THE PROCESS OF EVAPORATION IN A CLOSED CONTAINER WILL PROCEEDUNTIL THERE ARE AS MANY MOLECULES RETURNING TO THE LIQUID ASTHERE ARE ESCAPING.
AT THIS POINT THE VAPOUR IS SAID TO BE SATURATED AND THE PRESSUREOF THAT VAPOUR (EXPRESSED IN mmHg) IS CALLED SATURATED VAPOURPRESSURE.
IF THE LIQUID IS OPEN TO THE AIR, THEN THE VAPOUR PRESSURE IS SEEN AS
PARTIAL PRESSUREALONG WITH OTHER CONSTITUENTS OF THE AIR.
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BASICS OF DISTILLATION
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THE TEMPERATURE AT WHICH VAPOUR PRESSURE IS EQUAL TO THE
ATMOSPHERIC PRESSURE IS CALLED THE BOILING POINT.
SINCE THE VAPOUR PRESSURE IS LOW IN ORDINARY EVAPORATION AND THE
PRESSURE INSIDE THE LIQUID IS EQUAL TO ATMOSPHERIC PRESSURE PLUS
THE LIQUID PRESSURE, BUBBLES OF VAPOUR CAN NOT FORM INSIDE THE
BULK LIQUID.
AT BOILING POINT, VAPOUR PRESSURE IS EQUAL TO ATMOSPHERIC
PRESSURE, BUBBLES FORM AND VAPORIZATION BECOMES A VOLUME
PHENOMENA.
SINCE THE MOLECULAR KINETIC ENERGY IS GREATER AT HIGHER
TEMPERATURE, MORE MOLECULES CAN ESCAPE THE SURFACE AND THESATURATED VAPOUR PRESSURE IS CORRESPONDINGLY HIGHER.
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BASICS OF DISTILLATION
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VAPOUR PRESSURE OF A LIQUID AT A PARTICULAR TEMPERATURE IS THE
EQUILIBRIUM PRESSURE EXERTED BY MOLECULES LEAVING AND ENTERING
THE LIQUID SURFACE.
SOME IMPORTANT POINTS REGARDING VAPOUR PRESSURE:
ENERGY INPUT RAISES VAPOUR PRESSURE
VAPOUR PRESSURE IS RELATED TO BOILING
A LIQUID IS SAID TO BOIL WHEN ITS VAPOUR PRESSURE EQUALS THE
SURROUNDING PRESSURE
THE EASE WITH WHICH A LIQUID BOILS DEPENDS ON ITS VOLATILITY
LIQUIDS WITH HIGH VAPOUR PRESSURES (VOLATILE LIQUIDS) WILL BOIL AT
LOWER TEMPERATURES
THE VAPOUR PRESSURE AND HENCE THE BOILING POINT OF A LIQUID MIXTURE
DEPENDS ON THE RELATIVE AMOUNTS OF THE COMPONENTS IN THE MIXTURE
DISTILLATION OCCURS BECAUSE OF THE DIFFERENCES IN THE VOLATILITY OF
THE COMPONENTS IN THE LIQUID MIXTURE
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BASICS OF DISTILLATION
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THE BOILING POINT DIAGRAM SHOWSHOW THE EQUILIBRIUM COMPOSITIONSOFTHE COMPONENTS IN A LIQUID MIXTUREVARY WITH TEMPERATURE AT A FIXEDPRESSURE.
CONSIDER AN EXAMPLE OF A LIQUIDMIXTURE CONTAINING 2 COMPONENTS (AAND B) - A BINARY MIXTURE.
IN THIS EXAMPLE, AIS THE MORE VOLATILECOMPONENT AND THEREFORE HAS ALOWER BOILING POINT THAN B.
THE DEW-POINT IS THE TEMPERATURE ATWHICH THE SATURATED VAPOUR STARTS
TO CONDENSE. THE BUBBLE-POINT IS THE TEMPERATURE
AT WHICH THELIQUID STARTS TO BOIL.
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BASICS OF DISTILLATION
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FOR EXAMPLE, WHEN A SUBCOOLED LIQUID
WITH MOLE FRACTION OF A=0.4 (POINT A) IS
HEATED, ITS CONCENTRATION REMAINS
CONSTANT UNTIL IT REACHES THE BUBBLE-
POINT (POINT B), WHEN IT STARTS TO BOIL.
THE VAPOURS EVOLVED DURING THE BOILING
HAS THE EQUILIBRIUM COMPOSITIONGIVEN
BY POINT C, APPROXIMATELY 0.8 MOLE
FRACTION A.
THIS IS APPROXIMATELY 50% RICHER IN A
THAN THE ORIGINAL LIQUID.
THIS DIFFERENCE BETWEEN LIQUID AND
VAPOUR COMPOSITIONS IN EQUILIBRIUM ISTHE BASIS FOR DISTILLATION OPERATIONS.
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BASICS OF DISTILLATION
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DISTILLATION IS ACHIEVED BY THE APPLICATION AND REMOVAL OF HEAT.
THE SEPARATION OF COMPONENTS FROM A MIXTURE BY DISTILLATION DEPENDS ON
THE DIFFERENCE IN BOILING POINTS OF THE INDIVIDUAL COMPONENTS, CALLED
RELATIVE VOLATILITY.
WHEN A MIXTURE OF TWO OR MORE LIQUIDS IS HEATED AND BOILED, THE VAPOR HAS
A DIFFERENT COMPOSITION THAN THE LIQUID. FOR EXAMPLE, IF 10% MIXTURE OF
ETHANOL IN WATER IS BOILED, THE VAPOR WILL CONTAIN OVER 50% ETHANOL.
DISTILLATION IS BASED ON THE FACT THAT THE VAPOUR OF A BOILING MIXTURE WILL
BE RICHER IN THE COMPONENTS THAT HAVE LOWER BOILING POINTS. THE
CONDENSATE WILL CONTAIN MORE VOLATILE COMPONENTS.
AT THE SAME TIME, THE ORIGINAL MIXTURE WILL CONTAIN MORE OF THE LESS
VOLATILE MATERIAL.
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BASICS OF DISTILLATION
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THE VAPOR CAN BE CONDENSED AND BOILED AGAIN, WHICH WILL RESULT IN AN
EVEN HIGHER CONCENTRATION OF ETHANOL. DISTILLATION OPERATES ON THIS
PRINCIPLE.
REPEATED BOILING AND CONDENSING IS A CLUMSY PROCESS, HOWEVER, THIS CAN
BE DONE AS A CONTINUOUS PROCESS IN A DISTILLATION COLUMN.
IN THE COLUMN, RISING VAPORS WILL STRIP OUT THE MORE VOLATILE
COMPONENT, WHICH WILL BE GRADUALLY CONCENTRATED AS THE VAPOR CLIMBS
UP THE COLUMN
DISTILLATION CONSUMES ENORMOUS AMOUNTS OF ENERGY. BOTH IN TERMS OF
COOLING AND HEATING REQUIREMENTSIT CAN CONTRIBUTE TO MORE THAN 50%
OF PLANT OPERATING COSTS.
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HISTORY OF DISTILLATION
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USE OF DISTILLATION DATESBACKIN RECORDED HISTORY TO ABOUT 50 B.C.
INDUSTRIAL EXPLOITATION OF THIS SEPARATION PROCESS STARTED IN 12THCENTURYWHEN IT WAS USED TO PRODUCE ALCOHOLIC BEVERAGES.
BY THE 16TH CENTURY, DISTILLATION WAS BEING USED IN THE MANUFACTURE OF
VINEGAR, PERFUMES, OILSAND OTHER PRODUCTS.
EARLY DISTILLATION WAS BASICALLY BATCH STILLSTO PRODUCE ETHANOL.
NEXT WAS A SERIES OF STILLSWITH A CONTINUOUS FEEDTHAT FLOWED THROUGH
THE SERIES.
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HISTORY OF DISTILLATION
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HISTORY OF DISTILLATION
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HISTORY OF DISTILLATION
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THE NEXT PROGRESSION WAS TO PLACE THE STILLSIN A COLUMN ANDINTERCHANGE THE VAPOUR AND LIQUIDTO IMPROVE RECOVERY.
EARLY SMALL TOWERS HAD ONE OR TWO LARGE BUBBLE CAPS PERTRAY.
THE LIQUID AND VAPOUR TRAVELLED THROUGH THE CENTRE OFCOLUMN AND MIXED ON THE OUTER RING OF BUBBLE CAP.
LATER WHEN THE TRAY EFICIENCY BECAME MORE IMPORTANT, FLOWON THE TRAY BEGAN TO BE STUDIED AND SEVERAL TYPES OF TRAYSAND CHANGED FLOW PATTERNSWERE DEVELOPED.
TRAY COLUMNSAPPEARED IN THE SCENE IN THE 1820s ALONG WITHFEED PREHEATING AND USE OF INTERNAL REFLUX.
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HISTORY OF DISTILLATION
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TYPES OF DISTILLATION
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Based on system composition:
Binary distillation
Multi component distillation
Based on Operation Mode:
Batch distillation
Continuous distillation
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TYPES OF DISTILLATION
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Based on Extra feed:
Extractive distillation: Extra feed component leaves with thebottom product.
Azeotropic distillation: Extra feed component leaves with the top
product.
Based on Separation:
StrippingRemoves lighter from heavier component
RectificationRemoves heavier from lighter component
Fractionation
Separates lighter and heavier.Complex fractionationseparates into 3 or more components.
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VIDEOS
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A Sunrise Project for a Sunshine Future
PARADEEP REFINERY PROJECT
FacultyM ElamaranPNM-CG
Prepared byProduction-CG Team
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COMPONENTS OF FRACTIONATION EQUIPMENT
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Distillation columns are made up of several components, each of whichis used either to transfer heat energy or enhance material transfer. Atypical distillation contains several major components:
A vertical shellwhere the separation of liquid components is carried out.
Column internals such as trays/platesand/orpackingswhich are used toenhance component separations.
A reboilerto provide the necessary vaporization for the distillationprocess .
Acondenserto cool and condense the vapour leaving the top of thecolumn
Areflux drumto hold the condensed vapour from the top of the columnso that liquid (reflux) can be recycled back to the column
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COMPONENTS OF FRACTIONATION EQUIPMENT
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COMPONENTS OF FRACTIONATION EQUIPMENT
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The liquid mixture that is to be processed is known as the feedandthis is introduced usually somewhere near the middle of the columnto a trayknown as the feed tray. The feed tray divides the columninto a top (enriching or rectification) section and a bottom(stripping) section. The feed flows down the column where it iscollected at the bottom in thereboiler.
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COMPONENTS OF FRACTIONATION EQUIPMENT
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The condenser, reboiler and the column internal are termed asStages, where ideally vapour and liquid equilibrium is established, if
given enough residence time.
A vessel filled with a mixture of two components is said to be in
equilibrium if the number of light molecules escaping from theliquid is equal to the number returning to the liquid.
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REBOILER
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Heat is supplied to the reboiler to generate
vapour. The source of heat input can be any
suitable fluid, although in most chemicalplants this is normally steam. In refineries,
the heating source may be the output
streams of other columns. The vapour
raised in the reboiler is re-introduced into
the unit at the bottom of the column. Theliquid removed from the reboiler is known
as the bottoms productor simply, bottoms.
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REBOILER TYPES
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Types of Reboiler:
1. Vertical Thermo-Syphon Reboiler.
2. Horizontal Thermo-Syphon Reboiler
3. Kettle Reboiler.
4. Furnace
CONDENSER
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CONDENSER
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The vapour moves up the column, and as it
exits the top of the unit, it is cooled by acondenser. The condensed liquid is stored
in a holding vessel known as the reflux
drum. Some of this liquid is recycled back
to the top of the column and this is called
the reflux. The condensed liquid that isremoved from the system is known as the
distillateortop product.
CONDENSER TYPES
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CONDENSER TYPES
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A. Partial Condenser
The partial condenser is best used when there is alarge difference in the overhead vapour composition.For e.g. when there is a small amount of methaneand hydrogen mixed in a gasoline stream, the partialcondenser condenses the gasoline and leaves themethane and hydrogen as a vapour to be ventedfrom the overhead reflux drum.
B. Total Condenser
The total condenser is best used when there is smalldifference in the overhead vapour composition. Theoverhead vapour can be condensed at approximatelysame temperature.
CONDENSER TYPES
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CONDENSER TYPES
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C. Hot vapour bypass condenser
The hot vapour bypass condenser is best utilizedwhen there is a potential for large changes in the
overhead vapour composition. The vapourbypass can be used to maintain the pressure inthe tower system when the light components arelower than design.
COLUMN INTERNALS
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COLUMN INTERNALS
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TRAYS PACKINGS
TRAYS
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TRAYS
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Trays
A tray essentially acts as a mini-column, eachaccomplishing a fraction of the separationtask.
The more trays there are, the better is thedegree of separation.
Overall separation efficiency will dependsignificantly on the design of the tray.
Trays are designed to maximize vapour-liquidcontactby considering the liquid distributionand vapour distribution.
TRAYS
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TRAYS
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Vapour and liquid mixes to form a froth.
Mass transfer occurs within the froth.
The froth overflows across a weir.
The vapour disengages and move to thetray above.
The bulk liquid flows down the downcomer to next tray.
The steps are repeated in the next tray.
LIQUID AND VAPOR FLOW IN TRAY COLUMN
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LIQUID AND VAPOR FLOW IN TRAY COLUMN
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Each tray has 2 conduits called down
comers: one on each side. Liquid falls
by gravity through the down comersfrom one tray to the tray below.
A weir ensures there is always some
liquid (holdup) on the tray and is
designed such that the holdup is at a
suitable height, e.g. such that thebubble caps are covered by liquid.
Vapour flows up the column and is
forced to pass through the liquid via
the openings on each tray. The area
allowed for the passage of vapour on
each tray is called the active tray area.
LIQUID FLOW
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LIQUID FLOW
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Single pass Double Pass
TRAY TYPES
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TRAY TYPES
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1. BAFFLE TRAYS
2. SIEVE TRAYS
3. VALVE TRAYS
4. BUBBLE CAP TRAYS
5. HIGH EFFICIENCY TRAYS
TRAY TYPES BAFFLE TRAYS
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TRAY TYPES- BAFFLE TRAYS
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Baffle Tray
Baffle trays are trays of low foulingpotential with low efficiency. They haveopen areas approaching 50 % where ahigh efficiency tray will have open area
less than 15 %.
Three major types of baffle trays are
A. Shed decksB. Side to side traysC. Disc and Donut trays
TRAY TYPES SIEVE TRAYS
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TRAY TYPES- SIEVE TRAYS
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Flat perforated plate
Vapor issues from holes to givemulti-orifice effect
Simple in structure easy tofabricate
Turn down ratio is around 2:1
Pressure drop is moderate
Not suitable for operation undervariable loads
TRAY TYPES VALVE TRAYS
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TRAY TYPES- VALVE TRAYS
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Valve can be round or rectangularwith or without caging structure
Vary high capacity tray
Turndown ratio5:1
pressure drop Moderate (earlierdesigns somewhat higher recentdesign same as sieve tray)
Around 20%higher cost then sieve
tray
TRAY TYPES VALVE TRAYS
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TRAY TYPES- VALVE TRAYS
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CAGED VALVE
FIXED VALVE FLOATING VALVE
TRAY TYPES BUBBLE CAP TRAYS
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TRAY TYPES- BUBBLE CAP TRAYS
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Perforated plate with riser around
the hole
Cap in the form of inverted cups
over the riser
excellent turn down ratio
Good at extreme low liquid rates
Pressure drop is high
TRAY TYPES- BUBBLE CAP TRAYS
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TRAY TYPES- BUBBLE CAP TRAYS
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PACKED COLUMNS
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PACKED COLUMNS
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Random and structured Packed columns generate a mass transfer area by providing
a large surface area over which the liquid can transfer heat and mass to the vapour.
Packing utilizes a continuous contacting principle to separate products. A majoradvantage to packed column is the reduction in pressure across the column.
Typically the column pressure drop for a packed column is lessthan that of a trayed
column because of % open area.
Typical percent open areaof a trayed column is 8 to 15 %, whereas a packed columncan approach 50 %.Liquid accumulation for a packed column is lower than a trayed
column.
Another advantage of packed column is reduced foaming. Packing generates thin
films instead of fine dropletsfor mass and heat transfer, reducing entrainment when
foaming agents are present.
The introduction of vapour and liquid to the packing is very important. Trays will
normally equalize whatever mal-distribution is developed by vapour and liquid feeds.
PACKED COLUMNS
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PACKED COLUMNS
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TRAY VS PACKED COLUMNS
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TRAY VS PACKED COLUMNS
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Sl. No Description Trays Random
Packing
Structured
Packing
1 Pressure Drop High Low Lower
2 Fouling Service Yes No No
3 Feed Point Flexibility Yes Difficult Difficult
4 Liquid Hold up Considerable Small Small
5 Cleaning Easy Difficult difficult
6 Cost Low Low/Medium High
COLUMN PROBLEMS
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COLUMN PROBLEMS
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Foaming
Foaming refers to the expansion of liquid due to passage ofvapour or gas. Although it provides high interfacial liquid-vapourcontact, excessive foaming often leads to liquid buildup on trays.In some cases, foaming may be so bad that the foam mixes withliquid on the tray above. Whether foaming will occur dependsprimarily on physical properties of the liquid mixtures, but is
sometimes due to tray designs and condition. Whatever thecause, separation efficiency is always reduced.
COLUMN PROBLEMS
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COLUMN PROBLEMS
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Entrainment
Entrainment refers to the liquid carried by vapour up to the trayabove and is again caused by high vapour flow rates. It is
detrimental because tray efficiency is reduced: lower volatile
material is carried to a plate holding liquid of higher volatility. It
could also contaminate high purity distillate. Excessive
entrainment can lead to flooding.
COLUMN PROBLEMS
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COLUMN PROBLEMS
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Weeping/Dumping
This phenomenon is caused by low vapour flow. The pressureexerted by the vapour is insufficient to hold up the liquid on thetray. Therefore, liquid starts to leak through perforations.Excessive weeping will lead to dumping. That is the liquid on alltrays will crash (dump) through to the base of the column (via adomino effect) and the column will have to be re-started.
Weeping is indicated by a sharp pressure drop in the column andreduced separation efficiency.
COLUMN PROBLEMS
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COLUMN PROBLEMS
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Flooding
Flooding is brought about by excessive vapour flow, causing liquidto be entrained in the vapour up the column. The increasedpressure from excessive vapour also backs up the liquid in thedown comer, causing an increase in liquid holdup on the plateabove. Depending on the degree of flooding, the maximumcapacity of the column may be severely reduced. Flooding is
detected by sharp increases in column differential pressure andsignificant decrease in separation efficiency.
QUERIES
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QUERIES
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