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Uni t no : 4

Instrumentation for distillation

columns

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Ins t rumentation for dis t i l lat ion co lumns :

A: Operat ion o f dis t i l lat ion column, manipulated and con tro l led

var iab les in d is t i l la tion c olumn co ntro l , instrumentat ion fo r f low

con tro l of dis t i l late, top and bo ttom compos i t ion con tro l , ref lux

rat io con tro l pressure contro l schemes .

B : Mater ial and energy balance of dist i l lat ion co lumn

(BOOK: LIPTAK – II Process c on trol )

L ip tak - Dis t i llat ion cont ro l and opt imizat ion .

k kr ishnaswamy ( Process contro l )

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Contents:

In t roduct ion

What is a Dist i l lat ion p rocess

Examples

Flow sheet symbols

Types of Dist i l lat ion co lumn

Dist i l lat ion equipm ent and operat ion

Dist i l lat ion contro l

C-M Pair ing of dist i l lat ion

Composi t ion con tro l (Product qual i ty co ntro l )

Inferr ing compos it ion from Temperature (Inferential con trol)

Column p ressu re contro l

Contents :

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In t roduct ion:

Distillation is a method of separating mixture of two or

more miscible liquids into their pure components

Separation of components from a liquid mixture through

distillation depends on the difference in boiling points of

individual components, also depending on concentrationof components present,

There for distillation process depends on vapor pressure

characterizes of liquid mixture.

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In t roduct ion:

Why do we need to separate di fferent componets o f a

m ixture .Give two examp les ?

Remove the unw anted impur i t ies.

Remove the substance that are harm ful for our heal th.

Obtain pu re sub stances.

Separate two di f ferent , bu t useful componets .

Examples:

Grain is separated from stalks ,whi le harvest ing

We fi l ter tea leaves after preparing it .

Milk o r cu rd is chu rned to separate the butter.

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In t roduct ion:

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Flow sheet symbo ls of Dist i llat ion colum n:

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TYPES OF DISTILLATION COLUMNS: Based on operation

1) Batch and.

2) Cont inuous columns.Batch Columns:

In batch operation, the feed to the column is introducedbatch-wise. that is, the column is charged with a 'batch'and then the distillation process is carried out. When the

desired task is achieved, a next batch of feed isintroduced .

Continuous co lumns:

In contrast, continuous columns process a continuous

feed stream. No interruptions occur unless there is a problem with the column or surrounding process units.They are capable of handling high throughputs and arethe most common of the two types. We shall concentrateonly on this class of columns.

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Types of cont inuous column:

Continuous columns can be further classified according

to:1) The nature of the feed that they are processing:

Binary column - feed contains only two components.

Multi-component column - feed contains more than two

components.

2)The number of product streams they have:

Multi-product column - column has more than two

product streams

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Types of cont inuous co lumn:

Where the extra feed exits when it is used to help

with the separation:

Extractive distillation - Where the extra feed

appears in the bottom product stream.

Azeotropic distillation - Where the extra feed

appears at the top product stream.

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The type of column internals:

Tray co lumn - where trays of various designs are

used to hold up the liquid to provide better contact

between vapour and liquid, hence better separation.

Packed column - where instead of trays ‘Packings'

are used to enhance contact between vapour and

liquid

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Basic dist i l lat ion equ ipment and operation :

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Basic dist i l lat ion equ ipment and operation :

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Main componets of d is t il lat ion colum n:

Vert ical shell : where the separation of liquid

components is carried out.Column: Column internals such as trays/plates which

are used to enhance components separation.

Reboiler: Reboiler is a special type of heatexchanger used to provide heat necessary for

distillation.

Condenser: The overhead vapor leaving the column

is sent to a cooler or condensers and is collected in

a Accumulator.

Ref lux: A reflux drum to hold the condensed vapor so

that the liquid reflux can be recycled.

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Basic operation :

Heat is supplied to the reboiler to generate vapour. The

source of heat input can be any suitable fluid, although inmost chemical plants 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. The liquid removed

from the reboiler is known as the bottoms product orsimply, bottoms.

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Basic operation :

The vapour moves up the column, and as it exits the top of

the unit, it is cooled by a condenser. The condensed liquid isstored 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 is removed

from the system is known as the distillate or top product.

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Types of cont inuous column:

Thus, there are internal flows of vapour and liquid within the

column as well as external flows of feeds and productstreams, into and out of the column.

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Binary d ist i llat ion:

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Binary d ist i llat ion:

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Dist i l lat ion con trols:

Why Distillation control ?

1) Product composition should be constant.

2) To increase throughput.

3) Enhancing the column stability.

4) Safety of men and machineries.The 3 main objectives of column control can be stated as:

1) To set stable conditions for column operation.

2) To regulate conditions in the column so that the

product(s) always meet the required specifications.3) To achieve the above objective most efficiently,

e.g. by maximizing product yield, minimising energy

consumption, etc.

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Evaluat ion of c on trol strategies:

Controlled variables Manipulated variable:

i ) Overhead composit ion i) Reflux f low rate.

i i ) Bo ttoms com pos i t ion i i ) Rebo i ler heat ing

medium f low rate.

i i i )Ac cumulator level i i i ) Dist i l late f low rate.

iv)Bo ttoms level iv) Bo ttom f low rate.

v)Column pressure v) Condenser f low

rate/vapour b ypass rate.

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Paring can fol low three general con tro l structures:

Energy balance control:

Reflux and reboiler heatant flow to control composition

thus fixing the energy inputs

Material balance contro l :

uses the dist i llate and bot tom produc t f lows to contro l

com pos i t ion ,thus f ix ing the overal l mater ial balance

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Compos i t ion Measurement:

The direct measurement of composi t ion is mo re

expensive, bu t also m ore accurate and versat i le than isind irect temperature measu rement. Interm ittent

analyzers, such as chromatog raphs (wi th cycle t imes of

a few m inu tes), are often p rov ided w ith dead time

com pensat ion for c losed-loop con tro l . The analyzerupdate time must always be less than the response t ime

of the process. For improv ed accu racy, one usual ly

measures the impur i ty concentrat ion in the contro l led

stream . This way the upsets caused by feed composi t ion

changes, tower pressure or eff ic iency variat ions can be

more accurately corrected.

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Compos i tion con t ro l:

A) Control with On-line Analyzer.

- high investment and m aintenance costs.

- large measu rement delay.

B) Tray Temperature Control.Lim i ted con tro l performance

C ) Inferential Control.

Produc t qual ity (composi t ion) is est imated from

measu red pro cess variables, and the est imate is

used for con tro l.

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Control with On-line Analyzer.

Feedback control:

Feed forw ard con trol :

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Inferent ial control : Temperature measu remen t

1.Uses easily measu re p rocess variables (T, P, F) to

inferent ia l method more di f f icul t to m easure quant i t ies

such as composi t ions and m olecular weight .

2. Can subs tantial ly reduce analyzer delay.

3 . Can be much less expensive in terms of capital and

operat ing costs .

4.Can p rov ide measu rements that are no t available any

other way.

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Tray Temperature Con tro l :

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If the feed composition and column pressure are

constant, temperature can be used as indirect measureof composition.

When the bot tom produc t composi t ion is contro l led, the

temperature sensor is located in the lower half of the

column, and when overhead compos i t ion is con tro l led,in the upper hal f.

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The temperature sensor should be located on a tray that

strongly reflects changes in composition. This means

a 1% change in the manipulated variable (reflux or steam

flows in Figure 5, p. 10) should result in a temperature

change of at least 0.1 ºC to 0.5 °C, and that this change should

be symmetrical, meaning that a 1% drop or rise in

these flows should result in approximately the same size ofdrop or rise in the temperature on the control tray.

• When two compounds of relatively close vapor pressures are

to be separated (for example normal butane from isobutene),temperature measurement is not sensitive enough to measure

composition. In such cases, two temperatures or a

temperature difference (say, between Trays 5 and 15) can be

used instead of a single sensor. This configuration can also

be used to eliminate the effects of column pressure variations

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Top compos i tion con t ro l:

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Bot tom compos i tion cont ro l:

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Column pressure contro l :

In contro l l ing the pressure of a co lumn, the key

pieces o f equ ipment are the condenser and theaccumulator.

First the overhead vapors enter the condenser

(part ial or to tal).

If the condensat ion is incomplete, the condenser is

cal led a “partial” condenser , and the overhead prod uct is

wi thdrawn in both vapor and l iqu id phases.

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Total condensers are usual ly designed for accum ulator

pressures up to 215 ps ia (1.48 MPa) at an operatingtemperature of 120 ˚F (49 ˚C).

A part ial condenser is usual ly used between 215 ps ia and

365 ps ia (1.48 to 2.52 MPa), and refr igerant coo lant, suchas propane, is used if the operat ing pressure is g reater

than 365 psia (2.52 MPa). The condenser p ressure drop

is u sually about 5 ps ia (34.4 KPa).

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Most distillation columns are operated under

constant pressure, because at constant pressure,temperature measurement is an indirect indication of

composition,

but f loating the operating pressu re can have

advantages in many app l icat ions . When thecolumn pressure is allowed to float, the composition

must be measured by analyzers or by pressure

compensated thermometers.

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• The primary advantage of floating-pressure

control is that one can operate at minimumpressure, and this reduces the required heat input

needed at the reboiler.

• Other advantages of operating at lower

temperatures include increased reboiler capacityand reduced reboiler fouling.

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• In the following paragraphs, floating and constant

pressure control strategies will be described foroperations when

1) Liquid distillate is withdrawn in the presence

of non condensables.

2) Vapor distillate is withdrawn in the presence

of non condensables .

3) Liquid distillate is withdrawn when the amount of

non condensables is negligible.

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Coo l ing Water Con tro l , Negl ig ible Inerts:

In distillation processes where the distillate is in the

liquid phase and the quantity of inerts is negligible,the column pressure is usually controlled by

modulating the rate of condensation in the

condenser.

• In the control system shown for Column A in Figure10 (p. 18), the column pressure is controlled by

throt t ling the coo l ing water f low through the

condenser. This control scheme is recommended

only when the cooling water is treated, because

condenser tube fouling due to high temperature rise

across the tubes can occur.

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Coo l ing Water Con tro l , Neg l igib le Inerts :

• The advantage of this configuration is simplicity and low

maintenance cost, because the control valve is on the• water side and gives acceptable control performance,

provided the condenser is the bundle type, and the

cooling water is flowing through the tubes at a rate over

4.5 feet per second (1.35 m/s), or the water hasresidence time of less than 45 seconds.

• With such short residence time, the pressure controller

requires only a narrow proportional mode.

• As the residence time increases, the controller willrequire wider and wider throttling ranges and will also

need the addition of an integral mode to compensate for

the load changes.

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Coo l ing Water Con tro l , Neg l igib le Inerts :

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Liquid Dist i l late w ith Inerts:

The problem of pressure control can be complicated

by the presence of large quantities of inert gases,which must be removed, because otherwise they will

blanket the condensing surface and cause the

loss of p ressu re con tro l .

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Dist i l late w ith Inerts:

When the amount of inerts is variable, the purge

stream has to be modulated. In the control systemshown for Column B in Figure 10, as the inerts build

up in the condenser , the pressu re con trol ler will

first open up the con tro l valve (PCV-1), which

lowers the flooding level in the condenser and, byincreasing the heat transfer surface area, also

increases the rate of condensation.

When PCV-1 is nearly fully open (say 95%), thevalve position controller (VPC-2) is activated. Its set

point is 95%, so when PCV-1 has opened to 95%, it

starts purging the inerts by opening control valve

(VPCV-2).

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Liquid Dist i l late w ith Inerts :

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Vapou r dis t i l late w ith inerts:

The control system shown for Column C in Figure 10

is used when the distillate is in the vapor phase andinerts are present. As the overhead product is

removed under pressure control, the system

pressure will quickly respond to changes in the

distillate vapor flow .

Column p ressure with Vapour d ist i l late w i th

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Column p ressure with Vapour d ist i l late w ith

inerts p resents:

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• Here, the level of the overhead receiver regulates

the condensate generated by throttling the coolingwater supply.This way it will condense only enough

material to provide the column with reflux.

This control system will give acceptable performance

only if the condenser residence time on the

coolant side is short.

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If this is not the case, the cooling water flow should

be held constant, and the control system shown forColumn D in Figure 10 should be used. In this case,

some liquid condensate is sent to a small vaporizer,

which flow is regulated by the accumulator level,

and the resulting vapors are mixed with the vaporsfrom the pressure-control valve.

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Vapour Recompression p ressu re contro l :

Diagram :

Working:

Cv and MV:

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TH NK YOU

FOR YOUR TTENTION

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Unit no : 5

Ins trumentat ion fo r Evapo rators &

Crystal l izer

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Evaporators

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Contents:

Introduct ion

Single effect evapo rator

Mult i effect evaporator

Types of evaporator

Different p arameters in Evapo rator

Var ious Instrumentat ion schemes for the evaporator

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A :Types and operat io n o f ev aporato rs , Con t ro ll ed and man ipu lated

var iab les in evaporato r contro l p rob lem , ins trumentat ion for

feedback , f eed -f orward, cascade contr ol s trateg ies for evapo rator s ,

types and operat ion of c rys tall izers , con tro l led and man ipu lated

var iab les in c rys tall izer cont ro l p rob lem , in st rumen tat ion for contr ol

o f var ious t ypes o f c rys tal l izer s.

B : Selec tio n o f dev ices required in in strumen tat io n

I t d t i

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Int roduct ion

• Evaporation is one of the oldest unit operation.

• It is the process of concentrating aqueous solution in aclosed vessel or group of vessel in which concentrated

solution is the desired product and indirect

heating(usually steam) is the energy source.

• Evaporation is an energy intensive process

• Its efficiency can be improved by increasing the number

of evaporator in series and some of the energy can be

recovered by vapor recompression

• The product concentration can be measured by a variety

of analysers,including density, conductivity

Fl h t b l f E t

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Flow sheet symbo ls of Evapo rator :

Si l ff t t i

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Single effect evapo rat ion :

M lt i f f t t

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Mult ief fect evaporato r:

T f t

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Types o f evapo rator :

1) Horizon tal tube evaporator

2) forced circu lat ion

3) Short tube vert ical

4) Long tube vert ical

5)Fal l ing f i lm evaporator 6) Agitated f i lm evaporator

H i t l t b t

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Horizon tal tube evapo rator :

• The liquid is evaporated at the outside of the tubes. It flows

from one tube to the other in form of droplets, jets or as a

continuous sheet.

Hori on tal t b e e apo rator

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• Due to the impinging effect when flowing from one tube to the

other the heat transfer is higher compared to vertical falling

film evaporators.

• In addition this unit type can be operated with even lower

pressure drops compared to the vertical design.

•

It is also possible to design a higher heat transfer area for agiven shell compared to the vertical units. Perforated plates

or specially designed spray nozzles can be used in order to

guarantee a even liquid distribution for each tube.

• Cleaning of the outside tubes can be difficult, therefore this

type of evaporators is not used for processes with tendency

to foul.

• Tube dimensions are typically 0.75 to 1''.


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Feed

Distributor

Steam

Condensate

ConcentrateConcentrate

Vapour

Forced Circulation evapo rator :

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Forced Circulation evapo rator :

a Disadvantages:

i ) High cost

I i i ) High residence t ime

i i i) High operat ing cost

Short tube vert ical evapo rator :

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Short tube vert ical evapo rator :

Appl icat ion:

i) Sugar industry

Contro l :

i ) Level con trol

Long tube vertical evapo rator : RFC

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APPLICATION:

a)Black l iquor in the pulp and paper industry.

b) Corn syrup in the food

indust ry .

Contro l :

Level

Fal l ing f i lm vert ical evapo rator:

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In falling film evaporators the

liquid feed usually enters theevaporator at the head of the

evaporator.

In the head, the feed is evenlydistributed into the heating

tubes.

A thin film enters the heatingtube and it flows downwards

at boiling temperature and is

partially evaporated.


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The liquid and the vapor both flow downwards in a parallel flow.

This gravity-induced downward movement is increasingly augmented by theco-current vapor flow.

The separation of the concentrated product from its vapor takes place in the

lower part of the heat exchanger and the separator.

In most cases steam is used for heating.

Falling film evaporators can be operated with very low temperature

differences between the heating media and the boiling liquid.

They also have very short product contact times, typically just a few seconds

per pass.These characteristics make the falling film evaporator particularly suitable

for heat-sensitive products, and it is today the most frequently used type of

evaporator.


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Variables in evaporator:

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Manipulated variables:

i) Flow rateii) Flow rate of steam

Contro l led variables:

i ) Density of liquid

ii) Temperature

iii) Liquid level in evaporator

iv) Steam enthalpy

Disturbance: i)Variation in steam flow

ii)Disturbance in case of steam run out

iii)Disturbance due to noisy measurement signal

Evapo rator contro ls m ethods: ( Inst ru and con tro l )

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Evapo rator contro ls methods: ( Inst ru and con tro l )

The control systems are mainly to achieve the final product

concentration.Method used to measure the product density.

Common methods are:

A) temperature rise, boiling point rise

B) Conductivity

C) Differential pressure

D) Gamma gauge

E) U Tube densitometer

F) Buoyancy float

G) Refractive index

Why use Densi ty measu rement ?

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Why use Densi ty measu rement ?

Real time process control.

On l ine measurement Low maintenance and high rel iabi l i ty

Measurement of dif f icul t and extreme pro cess mater ials.

True densi ty measurement

Low total insta l lat ion co st.

Evapo rator contro ls m ethods: ( Inst ru and con tro l )

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Evapo rator contro ls methods: ( Inst ru and con tro l )

i ) Feedback contro l of evaporator

i i ) Cascade con trol o f evapo rator

i i i )Select ive control o f evapo rator

iv) Feed forward evapo rator contro l

Feedback contro l system o f evapo rator :

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Feedback contro l system o f evapo rator :

Cascade contro l of evapo rator :

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Cascade contro l of evapo rator :

Select ive contro l of evapo rator :

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Select ive contro l of evapo rator :

Product dens i ty measu rement: ( Methods )

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i )Temperature rise, bo i l ing po int ris e

i i ) Conduc t iv i ty

i i i ) Differential pressure

iv) Gamma gauge

v) U-Tube dens itometer vi) Buoyancy f loat

vi i) Refract ive index

Note:

Page no 1289 L ip tak II


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Crys tall izer controls

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Crystal l izer


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yp

Crystal l izat ion:

is a uni t operation that involves the separat ion ofa solute f rom i ts so lut ion in the form of crysta ls .

Crystall ization usual ly invo lves : Concentration ofsolution and cooling of solution until the solute

concentration becomes greater than its solubility at

that temp then the solute comes out of the solution

in the form of pure crystals

Flow sheet sym bo ls of Crys tal izer

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y y

Crystal l izat ion

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y

Crystallization is the unit operation that is best suitedto the recovery of dissolved substances fromsolutions

solid product has many desirable properties suchhas good flow charetercistes, handling easy, suitablefor packing

Methods to achieve crystallization:

i) Cooling

ii) Evaporation

iii) Vacuum or reactionThe choice of method depends upon the feed liquidcomposition, product specification ,and other engg consideration

Crys tall izat ion process(operat ion ):

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y p ( p )

Three basics steps:

i) Ach ievement of super saturat ion i i ) Formation of nuc lei

i i i) Grow th o f nuclei into c rys tal

Crys tall izat ion process(operat ion ):

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y p ( p )

Super saturation :

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p

Degree of freedom of c rys tall ization :

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g y


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y

Three methods:

i) I ndirect heating Crystal l izer

i i) Submerged combustion

ii i ) Spray evaporation

I ndirect heating Crystall izer :

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I ndirect heating Crystall izer :

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Submerged combustion:

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Thanks

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