Fundamentals of distillation

8/12/2019 Fundamentals of distillation

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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

PARADEEP REFINERY PROJECTA Sunrise Project for a Sunshine Future

Introduction

Basics of distillation

History of distillation

Types of distillation

Videos

Fractionating equipments


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INTRODUCTION TO REFINERY


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


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


1. Combination.

2. SeparationDistillation comes under this.

3. Reaction.


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BASICS OF DISTILLATION


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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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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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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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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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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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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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


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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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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TYPES OF DISTILLATION


Based on system composition:

Binary distillation

Multi component distillation

Based on Operation Mode:

Batch distillation

Continuous distillation


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TYPES OF DISTILLATION


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


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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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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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


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


Types of Reboiler:

1. Vertical Thermo-Syphon Reboiler.

2. Horizontal Thermo-Syphon Reboiler

3. Kettle Reboiler.

4. Furnace

CONDENSER


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CONDENSER


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


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


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


TRAYS PACKINGS

TRAYS


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TRAYS


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


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


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


Single pass Double Pass

TRAY TYPES
http://www.separationprocesses.com/Operations/Fig034b.htmhttp://www.separationprocesses.com/Operations/Fig034a.htm


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TRAY TYPES


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


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


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


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


CAGED VALVE

FIXED VALVE FLOATING VALVE

TRAY TYPES BUBBLE CAP TRAYS


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TRAY TYPES- BUBBLE CAP TRAYS


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



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PACKED COLUMNS


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PACKED COLUMNS


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


TRAY VS PACKED COLUMNS


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TRAY VS PACKED COLUMNS


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


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


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


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


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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Documents

Fundamentals of distillation