instrumentation and control of distillation towers

7/28/2019 Instrumentation and Control of Distillation Towers

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INDUSTRIAL PROCESS

CONTROL:

CONTROL OFDISTILLATION TOWERS


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Distillation separates mixture on the basis of a

difference in composition between liquid andvapour formed from the liquid.

In the process industry, distillation is widelyused to isolate and purify volatile materials.


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Distillation Equipment

FeedPump

Preheat

er

Column

The tower or columnhas two purposes:1) It separates the

feed into vapourportion whichascends the

column, and intoliquid portion whichdescends thecolumn.

2) Achieves intimatemixing between the

two counter currentflowing phases. Purpose of mixing is

to get an effectivetransfer of the morevolatile componentsinto the ascending

vapour and acorresponding


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If the feed is liquid, thetemperature at which itfirst starts to boil iscalled the bubblepoint.

If the feed is vapour,the temperature atwhich it first starts tocondense is called thedew point.

The column is

operated in atemperature rangethat usuallyintermediate to the twoextremes of dew pointand bubble point.

For effectiveseparation of the feed,

FeedPump

Preheat

er

Column


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The overhead vapourleaving the column issent to a cooler, andcollected as a liquid inreceiver. A part is withdrawn as

product or distillate. Remaining part is

returned to thecolumn as reflux.

The bottom liquid

leaving the column isheated in a reboiler. Part of the liquid is

withdrawn as bottomproduct or residue.

Remaining part is

vaporized andinjected into the

FeedPump

Preheat

er

Column

Reflux

Condens

er

Accumulator

Distillate

Reflux

Pump

Reboiler

Boil-up

Residue

This interaction

between the counter

flow of vapour and

liquid phase gives the

perfect separation of

volatile components

from liquid to vapour.


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Variables to Control

Independent variables are:

Column pressure.

Feed flow rate.

Feed composition. Feed temperature.

Reboiler (boil-up) control.

Reflux control.

Other variables are:

Level control.

Depends on the performance of independent variable

controllers.


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Most distillation control systems are based

upon maintaining the column pressure at someconstant value.

Variation of pressure will upset the control system bychanging the equilibrium condition of the material.

The set-point of pressure is the compromise

between two extremes. The pressure must be high enough to cause condensation of

the overhead vapour by heat exchange with the coolingmedium usually water.

The pressure must be low enough to cause vaporization ofthe bottom liquid by heat exchange with the heating mediumusually steam.

Pressure Control


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Column Pressure Control with

Liquid Distillate and Inerts

Presence of largepercentage of inert gascomplicates the pressurecontrol problem.

Uncondensable inert mustbe removed otherwise itwill accumulate andblanket off the condensingsurface.

LICLT

PIC

PR

C

PT

Vent

Column pressure can be

controlled by manipulating theheat transfer area inside the

condenser. If the pressure is less,

this control valve is closed by

using PRC so that liquid level

inside the pipe line just above the

CV increases, which will decrease

the heat transfer area, so that

condensation rate is decreased

(and the column pressure is

increased).

If the rate of condensate is large, level of liquid in

accumulator increases, which is sensed by the level

transmitter and opens the control valve, so that the

liquid level remains constant.

As the pressure passed a certain operating point (due

to the presence of large inert gas), PIC opens this

control valve so that inert gas with very small portion of

vapour is removed (because, the density of inert gas is

very small, and ascends the condenser tube, which isnot condensed).

This control valve is an ON-OFF control valve.


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Liquid Distillate and Negligible Inert Most common. Maintenance costs are low because the valve is on

the water line.

For properly designed condenser, PRC need onlyproportional controller, as narrow throttling range issufficient.

As the residence time of the water increases, it will

increase the time lag of the system. Wide proportional band is not sufficient.

Need automatic reset to compensate for the load changes.

Control the condensate which will maintain the water rateconstant.

Keep the condenser 10 to 15ft below accumulator.

LICLT

PR

C

PT

Water

Here the heat transfer area is constant. Column

pressure is regulated by manipulating the rate ofcooling medium.


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Vapour Distillate and Inert

System pressure will

quickly respond to

changes in flow of

cooling medium. Condenser will

condense only enough

condensate to provide

the column with reflux.

Condenser require a

short residence time.

LIC

LT

PR

CPT

Water


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Vacuum System

For some liquid mixtures, the temperature

required to vaporize the feed would need to be

so high that decomposition would result.

To avoid this, it is necessary to operate thepressure below atmosphere.


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Liquid Distillate and Inerts

Control valve of airstream is opened by PT,so that air pressurebecomes greater thanthe accumulatorpressure.

Air stream is flowedthrough the ejector.

Column pressureincreases

LICLT

PIC

PR

C

PT

Air

Stea

m

Click here to know the

effect if column pressure

P1 is too small (or morevacuum).

Click here to know the

effect if column pressure

P1 is close to theatmospheric pressure

Control valve of airstream is closed byPT, so that airpressure becomeszero.

Column vapour isflowed through theejector.

Column pressuredecreases.

Measurement of accumulator

pressure (which is almost

equal to column pressure)introduces less time lag than

the direct measurement of

column pressure


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Only six variables need to be fixedinorder to predetermine all conditions ina distillation column. The column pressure can be the first controlled variable.

If fixed feed conditions are assumed,

three more independent variables aredetermined. Feed rate.

Feed composition.

Feed temperature.

Feed Control


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Feed Flow Rate

FRC FT

By holding the flow rate

constant, best means of

operation stability is

obtained.Narrow proportional band

is used.

Feed


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Feed Composition

Feed composition have greater influence on

operation.

Feed composition is seldom subject to

adjustment.Changes are made elsewhere.


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Feed Temperature

FRC FT

For efficient separation, itusually is desirable to have thefeed at its bubble point when itenters the column. Unless the feed comes directly

from some preceding distillationstep, an outside source of heatis required.

Constant temperature feeddoes not necessarily meanconstant feed quality.

If feed composition varies, itsbubble point also varies.

Set the temperature control at apoint which is equivalent to thebubble point of heaviest feed.

TT

TR

C

Steam

To Column

SP

Feed


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Reboiler (Boil-up) Control

Boil-up rate iscontrolled bysetting the flow ofheat to the reboiler.

The amount oflighter boilingmaterial in thebottom product isdetermined by the

setpoint. Setting more steam

will driven largeamount of lightermaterial as boil-up.

LICLT

FR

C FT

Steam


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Reflux Control

LICLT

PR

C

PT

Since distillation achievesseparation of materials bycounter currentlycontacting vapour andliquid, reflux furnishes thecontinuous supply of liquid

to the column. Level control shown is not

an independent variablebecause, it reflux thedifference between whatcomes into accumulator and

what goes as reflux.

PT

PR

C

FR

C FT


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Variable Column Feed

Any inadvertent changeswhich occurs in the firstcolumn would be reflectedin the quantity andcomposition of the feed to

the second column. Liquid level controllers can

have a wide proportionalband so that changes inthe reboiler / accumulator

level can swing over a widerange without drasticallyupsetting the flow ofproducts. Second column will receive

varying flow rate, whichmust be avoided.

LR

CLT

FR

CFT


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Thank You

instrumentation and control of distillation towers

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