distillation columns.pdf

7/24/2019 Distillation Columns.pdf

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Distillation Columns

Distillation can bedefined as a process inwhich a liquid or vapor

mixture of two or moresubstances is separatedinto its componentfractions of desired

purity, by the applicationand removal of heat


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Batch and Continuous Columns

One way of classifyingdistillation column

type is to look at howthey are operated. Thuswe have:

1. Batch Columns

2.

ContinuousColumns


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Batch ColumnsIn batch operation, the feed to the

column is introduced batch-

wise. That is, the column is

charged with a 'batch' and thenthe distillation process is

carried out. When the desired

task is achieved, a next batch

of feed is introduced.


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Continuous Columns

In contrast, continuous columns

process a continuous feed stream.

No interruptions occur unless there

is a problem with the column orsurrounding process units. They are

capable of handling high

throughputs and are the most

common of the two types and used

in SHARQ. We shall concentrate

only on this class of columns.


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Types of Continuous Columns

Continuous columns can be furtherclassified according to:

1. The nature of the feed that they areprocessing

2.

The number of product streamsthey have

3. Where the extra feed exits when itis used to help with the separation

4.

The type of column internals


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The nature of the feed that they

are processing

Binarycolumn - feed contains

only two components

Multi-componentcolumn - feed

contains more than two

components


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The number of product

streams they have Multi-productcolumn -

column has more than

two product streams


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Where the extra feed exits when it is used

to help with the separation

Extractivedistillation -

where the extra feed

appears in the bottom

product stream

Azeotropicdistillation -

where the extra feed

appears at the top productstream


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The type of column internals

Traycolumn- where trays ofvarious designs are used tohold up the liquid to provide

better contact between vaporand liquid, hence betterseparation

Packedcolumn- where insteadof trays, 'packings' are used to

enhance contact between vaporand liquid


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Tray

Columns


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Packed

Columns


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Main Components of Distillation

Columns A vertical shellwhere the separation of

liquid components is carried out

Column internals such as trays/platesand/or

packingswhich are used to enhance

component separations A reboilerto provide the necessary

vaporization for the distillation process

A condenserto cool and condense the vapor

leaving the top of the column

A reflux drumto hold the condensed vapor

from the top of the column so that liquid

(reflux) can be recycled back to the column


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Strippingsystems removelight material from a heavyproduct

Rectificationsystemsremove heavy materialfrom a light product

Fractionationsystemsremove a light materialfrom a heavy product and aheavy material from a lightproduct at the same time.


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Heat is supplied to the reboilerto generate vapor. The source

of heat input can be anysuitable fluid, although in mostchemical plants this isnormally steam

The vapor raised in the reboileris re-introduced into the unit atthe bottom of the column. Theliquid removed from thereboiler is known as the

bottoms productor simply,bottoms.


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The vapor moves up the column, and as it

exits the top of the unit, it is cooled by a

condenser.

The condensed liquid is stored in a holdingvessel 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 is removed from

the system is known as the distillateor top

product

Thus, there are internal flowsof vapor and

liquid within the column as well as external

flows of feeds and product streams, into and

out of the column


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Trays and Plates The terms "trays" and "plates"

are used interchangeably.

There are many types of tray

designs, but the most commonones are :

1. Bubble cap trays

2. Valve trays

3.

Sieve trays


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Bubble Cap Trays A bubble cap tray has riser or

chimney fitted over each hole,and a cap that covers the riser

The cap is mounted so that there

is a space between riser and capto allow the passage of vapor

Vapor rises through the chimneyand is directed downward by thecap, finally discharging through

slots in the cap, and finallybubbling through the liquid onthe tray


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Valve Trays In valve trays, perforations are

covered by liftable caps

Vapor flows lifts the caps, thus

self creating a flow area for thepassage of vapor

The lifting cap directs the vapor

to flow horizontally into the

liquid, thus providing better

mixing than is possible in sievetrays.


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Sieve Trays Sieve trays are simply

metal plates with

holes in them Vapor passes straight

upward through the

liquid on the plate


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Because of their efficiency,

wide operating range, ease

of maintenance and cost

factors, sieve and valve

trays have replaced the

once highly thought of

bubble cap trays in many

applications


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Liquid and Vapor Flows in a Tray

Column The next few figures

show the direction of

vapor and liquid flowacross a tray, and across

a column


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Fluids Flow Across Column


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Fluids Flow Across Trays


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Each tray has 2

conduits, one on eachside, called

downcomers

Liquid falls through the

downcomers by gravity

from one tray to the one

below it


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A weiron the tray ensures

that there is always some

liquid (holdup) on the tray

and is designed such that

the the holdup is at a

suitable height, e.g. suchthat the bubble caps are

covered by liquid


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Being lighter, vapor flows

up the column and isforced to pass through the

liquid, via the openings on

each tray

The area allowed for the

passage of vapor on each

tray is called the active tray

area


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Packings Packings are passive

devices that are

designed to increase theinterfacial area for

vapor-liquid contact


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These strangely shaped pieces

are supposed to impart good

vapor-liquid contact when a

particular type is placed

together in numbers, without

causing excessive pressure-

drop across a packed section

This is important because a

high pressure drop would mean

that more energy is required to

drive the vapor up thedistillation column


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Packings versus Trays

packings provide extra inter-

facial area for liquid-vapor

contact

efficiency of separation isincreased for the same column

height

packed columns are shorter

than trayed columns


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Packed columns are called

continuous-contactcolumnswhile trayed

columns are called staged-

contact columnsbecause of

the manner in which vaporand liquid are contacted


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COLUMN REBOILERS

There are a number of designs

of reboilers

they can be regarded as heat-

exchangers that are required totransfer enough energy to bring

the liquid at the bottom of the

column to boiling point


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Thermo-syphon Reboiler


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Kettle-Type Reboiler


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THANK YOU

Q&A

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