Mass Transfer Basics

7/27/2019 Mass Transfer Basics

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MASS TRANSFER OPERATION

BASICS

Presentation by

VMM (CTS)


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Mass transfer : Transfer of material from one homogeneous phase

to another. The driving force for mass transfer is concentration

difference or difference in activity coefficient.

Mass transfer operation involves changes in composition ofsolution & mixtures. Transfer of substance through another on a

molecular scale.

Mass transfer coefficient : Rate of mass transfer per unit area per unit

conc. difference. It depends on diffusivity,viscosity,density,velocity

& linear dimension D.

KcD

Dv= f (( DG / ) , ( / Dv ) )

Nsh = f ( NRe , Nsc )Why Mass transfer operation:

Any chemical process requires :

1) Purification of raw material.

2) Separation of product from byproduct.


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Gas-Liq.

DistillationAbsorption

Desorption

Humidification.

Dehumidification

Gas-Solid

SublimationDrying

Adsorption.

Liq-Solid

Crystallisation

Liq-liq

Extraction

Position of operating line relative to the equilibrium line decides1. Direction of mass transfer.

2. How many stages required for given separation.

Distillation Desorption Absorption

Eqm line

Operting line

Eqm line

Operting line

Operting line

Eqm line


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Design principle (MTO)

Equilibrium characteristics of system. Material balance.

Diffusional rate.

Fluid dynamics.

Energy requirement.

MTC : Regulate the rate at which equilibrium is approached.

Control time required for separation,size & cost of equipment.


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Basic equations & laws.

Yi Mole fraction of component i in vapor phase.

Xi Mole fraction of component i in liquid phase.

Pvi Vapor pr of comp. i

Pi Partial pr. Of component i

Pt Total pr.

k Henrys constant

Relative volatility

Yi

XiKi =

Pi = Pv * Xi

Pi = k * Xi

Pi = Yi * Pt =Ki

K j=

Pvi

Pvj

Henrys law

Raoults law

Distillation,Extractive distillation,Liquid-liquid extraction,Absorption

are all techniques to separate binary & multicomponent mix. Of

liquid & vapors.


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Distillation :

Vapor liquid equilibrium data

Predict (Thermodynamic

calculation)Experimentally calculate.

These data need to relate temp, pressure & composition.

Two types of system

Ideal (Raoults law)

Nonideal system

Accurate experimental data necessary.

For non-ideal system use of specific empirical relationship that predict

with varying degree of accuracy the vapor pressure ,concentration

relationship at specific temp. & pr.

Concept of fugacity & activity are fundamental to the interpretation

of non-ideal system.


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Distillation Types

1) Flash distillation.

2) Batch Distillation.

3) Steam Distillation.4) Azeotropic & Extractive Distillation

Batch Distillation :

Used for

1) Feed composition may change from batch to batch.

2) Negligible hold up in the column & condenser relative to that

in receiver & kettle.

Flash Distillation : Vaporising a definite fraction of the liquid in

such a way that the evolved vapor is in equilibrium with residualLiquid.


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3) Batches are relatively small where certain components are

separated in pure form heating a heavier residue.

Most Batch Distillation follow constant relative volatility vaporliquid equilibrium curve.

y =x

1 + x ( - 1 )

LogF Xf

W XS= Log

F ( 1Xf)

W ( 1XS )

F = D + W

F Xf = D Xd + W Xw

Feed

Bottom

Distillate


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Steam Distillation :

Possible to distill an organic compound at much lower temp.

At constant system pr.PT ,steam lowers the partial & vapor pressure of

organic compound & its corresponding boiling pt.

Due to immiscibility of water ,it can be separated from product by

simple condensation & followed by decanting.

Application : Purification of heat sensitive material as an alternative to

vacuum distillation.

Azeotropic & Extractive Distillation.

Very close boiling mix can be separated economically by this technique.

Solvent when added will increase the difference between volatilities of

light & heavy component. The attraction of solvent to one of the

component reduces the volatility of solvent & the component to which

it is attracted.


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Solvent for Distillation should be

Non-corrosive.

Should not react with feed to form undesirable product.

Non-toxic. Azeotropic solvent should have volatility near the major component

desired in overhead product & in Extractive distillation its volatility

should be lower than major component to be withdrawn at bottom.

Extractive Distillation :

An agent modifies the relative volatility between key component

Without forming an azeotrope.The agent is nonvolatile & called Solvent.

Extractive Distilation is simpler process than Azeotropic Distillation Solvent boils far above the system component.

Because of the low volatility the solvent always leaves the Extractive

column with the bottom product


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In Extractive distillation attraction of solvent to one of the component

Is attributed to

Hydrogen Bonding.

Polar characteristics of the solvent & members of the mix. Formation of weak unstable chemical complex.

Chemical reaction between solvent & one of the component.

y

x

Non-AzeotropeMin. boiling

azeotrope

Max. boiling

Azeotrope.

Azetropic Distilation


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Extractive & Azeotropic Distillation.

Water

Crude

MeOH

Acetone

MeOH

water

Oxide Impurities.

Acetonewater

water

pentane


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q = Moles of liquid flow in stripping section per mole of feed.

a) Cold feed q > 1

b) Saturated liquid q = 1c) Feed partially vapor 0 < q < 1

d) Feed at dew point q = 0

e) Feed superheated vapor q < 1 (a)

(b) ( c ) ( d )

( e )


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q = 1 +CpL ( TbTf)

q = - Cpv ( TfTd )

y =q

( 1q )

x +xf

( 1q )

X D

( RD + 1 )

XFXB XD

abc

d

e

Feed line

RD = Reflux ratio

Tf

Td

Tb

= Feed temp.

= Dew pt.

= Bubble pt.

CpL = Sp heat of liq.

Cpv = Sp heat of vapor


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Feed line

X D

( RD + 1 )

XB XFXD

McCabe - Thiele


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Fenske Equation

Nmin =

Log ( XD ( 1XB ) / ( XB ( 1XD )

Log AB

1.0

Reflux ratio =L

D=

Reflux flow

Distillate rate

Eq of operating line of rectifying section.

yn+1 =

RD

( RD + 1 )xn +

( RD + 1 )

XD


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Minimum reflux ratio : No of plates infinite.

Total reflux : Minimum no of plates

C.S. area of column . Flow rate of vapor.

As the reflux ratio increases the vapor & liquid flow for given

production rate increases

Total cost

Cost of heating & cooling

Fixed charges

Reflux ratio.Rm

Annual

cost

Total cost Total plate area.

No of plates * C.S area of col

Optium RR = (1.1-1.5) Rmin.


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Distillation:

Tower dia 0.3 m - 9.0 m

No of plates few to 100.

Plate spacing : 6 inch to several feet.Temp : upto 900 deg C.

Material distilled : Viscosity,diffusivity,corrosive nature,tendency to

foam,complexity of composition.

Most plant operate at R.R. somewhat above the optimum.Total cost

is not very sensitive to R.R. in this region & better operating flexibility

is obtained.

70% to 80% of C.S area is used for bubbling / contacting.

The vapor velocity should be high enough to create a frothy mix. of

liquid & vapor that has a large surface area for Mass transfer.

Misoperation such as weeping,foaming,entrainment,flooding

Short circuiting,poor vapor distribution should be avoided.


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Selection of contacting device is based on

Tray type / packed type

Vapor handling capacity. Liquid handling capacity.

Mass transfer efficiency.

Flexibility for wide range of operation.

Pressure drop.

Cost.

Flooding pt.

At flooding pt. Liquid continues to flow down the column,but builds

up at greater rate from tray to tray. Flooding is associated with highliquid load over a wide range of vapor rates.Foaming tendency of

the liquid influence the flooding.


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Entrainment :

When mist & liquid particles carry up in the vapor from the liquid

from one tray to tray above,sufficient tray spacing should be available

to prevent entrainment.

Tray pressure drop :

For normal operation pressure drop per tray

Pressure Vacuum

24 inch water 24 mmHg

Tray stability :

A tray is stable when it can operate with acceptable efficiencies under

condition that fluctuate, pulse or surge.

Turndown ratio :

Ratio of max. allowable vapor rate at or near flooding condition to

the min. vapor rate when weeping or liquid leakage becomes significant.


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Plate efficiency :

Plate efficiency is a function of rate of mass transfer between liquid

& vapor. Any mis-operation of column such as excessive foaming

or entrainment ,poor vapor distribution,or short circuiting ,weeping,

dumping of liquid lowers the plate efficiency.

Overall efficiency : Ratio of no. of ideal plates needed in the entire

column to the no. of actual plates.

Murphee sfficiency : The change in vapor composition from one plate

to the next divided by change that would have occurred if the vaporleaving were in equilibrium with liquid leaving.

Minimum reflux ratio.

At this reflux ratio desired separation is just possible but infinite no. ofplates is required.This min. reflux ratio is guide in choosing a

reasonable R.R. for an operating column & in estimating no. of plates

needed for given separation at certain value of R.R.


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Bubble cap Sieve tray

Capacity : Moderately high Higher than bubble cap at design

at low throughput performance

drops as efficiency falls.Efficiency : High As high as bubble cap in the

region of design, falls to

unacceptable value

when capacity reduces < 60%Entrainment : Three times 1/3 rd of bubble cap tray.

that of sieve tray

Flexibility: Most flexible design Not suitable for column operating

for high & low liquid rate. under variable load,falling < 60%Allows positive drain of of design.Tray weeps liquid at

liquid from tray.Liquid low vapor rate.

head is maintained by weir.


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Bubble cap Sieve tray

Tray spacing : 18 inch avg. Can be closer than bubble cap

24 to 36 inch for Vac condition due to improved entrainment15 inch avg.

9,10,12 inch acceptable

20,30 inch for vacuum.

Application : All service except Systems where high capacityextremely coking,polymer design rates to be maintained.

formation or other high Handles suspended solid

fouling condition. Particles.


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Factors to Consider When Selecting

High Performance Trays Capacity & Hydraulic Limitations

Pressure Drop

Efficiency

Operating Range

Resistance to Fouling

Existing Column Configuration

Equipment Cost / Installation Cost


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Various High Capacity / High Performance

Trays available in the MarketNorton Triton Trays

Nutter MVG trays

Koch-Glitsch Nye Trays

Koch-Glitsch MaxFrac Trays

Koch Glitsch Superfrac Trays

UOP E-MD trays

SHELL High capacity trays


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Saint-Gobain Norpro High Capacity Triton Trays increase the

capacity of towers by increasing the area available for liquid / vapour

contacting over conventional tray designs.


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Conventional Tray High Capacity Triton


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Conventional Tray High Capacity Triton

Trays

Spray Height Profiles


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General characteristics

Trays : Usually made of sheet metals of special alloys. Thickness is

governed by corrosion rate. Trays must be stiffened & supported &fastened to the shell to prevent movement owing to to surge of gas,

With allowance for thermal expansion. Large trays must be fitted with

Man-ways.

Tray spacing :

It should be such that insurance against flooding & excessive entrainment

where tower height is important consideration.For all except smallest dia.

tower ,20 inch spacing is considered. For small dia tower spacing of

6 inch is considered.

Tower diameter.: Tower dia should be sufficiently large to handle the

gas & liquid rates within the region of satisfactory operation.


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FloodingWeeping

Coning

DumpingPriming

Excessive entrainment

Satisfactory

operation

Gas rate

Liquid rate

Vf = Cf( L -G )

L

Cf = Empirical constant

Vf = Superficial gas velocity.

Tower dia can be decreased

by use of increased traySpacing ,so that tower cost

which depends on height

& dia. passses through min.

at some optimum tray spacing.


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Downcomer :Liq. Is fed from one tray to next by downspot .Adequate

Residence time must be allowed in the downspot to permit disengaging

Gas from liquid so only clear liquid enters the tray below.

Weirs : It maintains the depth of liquid required for gas contacting

on the tray. In order to ensure uniform distribution of liquid flow on a

Single pass tray a weir length of 60 to 80% of tower dia is used.

Liquid flow :

Small tower : Reverse flow.

Most common : Single pass cross flow.

Commertial col. Upto 15 m dia have been build.

Two pass trays are common for dia of 3 to 6 m & more passes for

larger dia.

Reverse flowSingle pass

cross flow


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Process Requirement of col. Internals.

A) Primary Requirement.

a) Efficiencyb) Capacity.

B) Secondary Requirement :

a) Low pressure drop.

b) Resistance to fouling.c) Resistance to corrosion.

Type of internals :

TraysRandom packing.

Structured packing.


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Tray :

Sieve

ValveBubble cap.

High capacity trays.

Features :

Good efficiency.

Good capacity

Good resistance to foulingReliable

Relatively high pressure drop.

Random packing.

1st generation Rasching ring,saddles

2nd generation Pall ring

3rd generation Properietary packing

Features : More capacity or more efficiency than trays.

Low delta p than trays, Excellent revamp tool.

Easy installation, Sensitive to vapor liquid maldistribution.

Price higher than tray.


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Structured packing

Proprietary : Gempack (Glitch)

Flexipack ( Koch)

Mallapack (Sulzer)Intalox (Norton)

Feature

More capacity & more efficiency than trays.Very low pressure drop.

Sensitive to vapor liquid maldistribution.

Unreliable for high pressure distillation.

High price.


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Packed tower Tray tower

1) Gas pr drop Smaller.

2) Liquid hold up Smaller.3) Liquid/gas ratio High values can be Low values can be

best handled best handled

4) Liquid cooling Cooling coils more

readily built.

5) Side stream More readily removed.

6) Foaming system Operate with less

bubbling of gas &

hence more suitable.

7) Corrosion For difficult corrosionless costly.

8) Solid present. Not satisfactory. Not satisfactory.

9) Cleaning. Frequent cleaning difficult Easier.

10) Temp fluctuation Fragile packing tend to

crush.


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Control philosophy :

Component segregation is achieved by control of heat load.

Stream splitting is achieved by control of product flow.

Column pr too highFeed system may be unable to input feed

to column.Allowable design pr may exceed.

Column pressure too low : Product may not flow from system.

Too high temperature (bottom)Product degradationexcess over equipment design.

Rapid variation in flow or pr.Control of d/s, u/s equipment

become impossible.

Pressure control :

Pr can be controlled by manipulating vapor product or noncondensible

vent stream.

To get constant top vapor product composition,condenser outlet temp.

needs to be controlled.


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Vary the condenser cooling by manipulating the vapor product or

non-condensable vent stream.

Pr can be controlled by variable HTC in condenser. Condenser must

have excess surface. Vacuum condenser (variable HTC). Very the amount of inerts leaving

the condenser.

Hot gas bypass.

Reflux drum

PT

PT

To flare

Column


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Temperature control : Measure of composition.

Temp is controlled either at top or bottom depending on which

product specification is important.

For high purity operation temp. will be controlled at intermediate pt.The pt. Where dT/dC is max. is the best place to control temp.

Column top temp. Controlled by manipulating reflux.

For partial condenser it is typical to control condenser outlet temp.instead of column top temp.

Column bottom temp. R/b outlet line temp. is used for controlling.

For smooth control cascade arrangement ,FRC on heating medium line

Set pt. Of which is manipulated by TRC.

Feed temp. This is critical as this decides the vapor & liq flow in feed

zone. Often amount of heat available in the bottom is close to the

optimum feed preheat.

L l l


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Level control :

For total condenseraccumulator level is controlled by varying

distillate draw.

For partial condenser ,it can be controlled with condenser hot gasbypass.

Bottom level is controlled by bottom draw.

Flow control

Feed flow often not controlled.Liquid product flow are on level than flow control.

Top vapor product is usually on pr. control.

Reflux is frequently on FRC , but also may be on column TRC or

accumulator level.

Differential temp. control : Used to control the column traffic.

Good way is let the differential pressure control the heating medium

to the r/b. Largest application is in packed tower where it is desirable

to run at 80 to 100% of flood for best efficiency.


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Differential temp. control :

Diff temp control across several bottom section trays is key to

maintain purity control.

Column side draw flow was put on control by critical temp. difference. This controls the liquid reflux running down to critical zones by

varying the liquid draw off at side draw.

Optimisation:

Hold reflux at minimum to deliver distillate purity.

Boil up at minimum to deliver bottom purity.

Higher throughput or improved product quality is balanced against

higher operating cost such as labor energy or maintenance.


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R/B selection :

Forced circulation.

Natural circulation.

Vertical thermosyphon.

Horizontal thermosyphon.

Flooded bundle.

Horizontal thermosyphon : Used for larger duties,dirty process &frequent cleaning is required. Process is usually on shell side.

Vertical thermosyphon : Used for smaller duties,clean process &

vaporisation < 30%. Viscosity of r/b feed should be < 0.5 cp.

Forced circulation : Usually used where piping pr drop is high &

Natural circulation is impractical.

Kettle : Very stable & easy to control. No two phase flow ,expensive.


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Dont use lower pr. Than desired,because separation efficiency

& throughput decreases as pressure decreases.The requirement of bottom temp. to avoid overheating heat sensitive

material may become predominant.

Low pressure operation requires larger dia. Column.

f i i f


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Start

Distillate & bottom

composition known.

Calculate bubble pt pr

Pd at 49 deg C.

Calculate dew pt pr Pd

at 49 deg C

Choose a refrigerant so as

to operate partial condenser

at 415 psig

Estimate

bottom pr.

Pb

Pd < 215 psia

Pd < 365 psia

Pd > 365 psia

Pd > 215 psia

Lower pr. Pd

appropriately

Calculate bubble pt.

Temp. Tb of bottom

at Pb

Tb < bottomtemp. or

Critical temp.

Method of estimation of col pr.

ETHERMAX PROCESS


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TI

TI

TRC

REACTOREFFLUENT

26

30

15

50

FRC

MP STEAM

CONDENSATE

TRC

COOLINGWATER

LIC

TAME PRODUCTTO STORAGE

COOLINGWATER

PRC

COOLINGWATER

FR

TO RAFFINATEWASH COLUMN

FR

SUMMER

LIC

ETHERMAX PROCESS

PROCESS FLOW

E t ti It i th ti f th tit t f li id l ti


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Extraction : It is the separation of the constituent of a liquid solution

by contact with another liquid.

Feed : Solution which is to be extracted.

Solvent : Liquid with which feed is contacted.Raffinate : Residual liquid from which solute has been removed.

Distillation & Evaporation are direct separation method,the product

of which is pure substance.

Liquid Extraction produces new solution which must be separated by

Distillation or Evaporation.

Extraction :

1. Extraction is attractive alternative to Distillation under high vacuum

& low temp. to avoid thermal decomposition.2. Liquid Extraction incurs no chemical consumption or by-product

formation & less costly.

3. Aromatic & paraffinic HC of nearly the same MW are impossible to

separate by distillation due to vapor pr nearly same can be easily

separated by Extraction.


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Variables which influence the performance of Extraction.

1) Liquid System.

Chemical identity & corresponding physical property.

Concentration of solute. Direction of Extraction

a) Aq. To organic. b) Continuous to dispersed phase

Total flow rate of liquid.

Ratio of liquid flow.

What liquid is dispersed.

2) The Equipment.

Design Packed / Mechanically agitated. Size shape of

packing,arrangement of baffle.

Nature & extent of mechanical agitation whether rotary orpulsating.

MOC,which influence relative wetting by the liq.

Height & end effect.

Diameter of extractor & extent of axial mixing.


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Solvent

Extract Feed

Raffinate

A

RS

E

C : Solute in Feed A to be separated

by extracting solvent S.

Effect of temperature :Solubility of feed & solvent increases with increasing temp. & above

certain temp. t1, they dissolve completely. Liquid extraction operation

which depends on formation of insoluble liquid phase must be carried

out < t1


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Effect of Pressure :

Except at very high pressure the influence of pressure on liquid

equilibrium is so small that it can be generally ignored.

Choice of Solvent :

1) Selectivity : For all Extraction operation selectivity must exceed

unity,the more so better.If selectivity is unity no separation is possible.

Selectivity =(wt. fr. of C in E) * ( wt fr. of A in E)

(wt. fr. of C in R) * (wt. fr. of A in R)

2) Distribution coeff.Ratio of y* / x

Higher the value ,less solvent will be required for Extraction.

3) Insolubility of solvent : Solvent which is more insoluble is


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3) Insolubility of solvent : Solvent which is more insoluble is

preferable.

4) Recoverability : It is necessary to recover the solvent for reuse.

5) Density : Difference in density of saturated liquid phase is necessary,

for stepwise & continuous contact equipment operation larger this

difference better.

6) Interfacial Tension : Larger the interfacial tension more readily

coalescence of emulsion will occur but more difficult the dispersion

of one liquid in the other. Coalescence is usually of great importance

hence Interfacial tension should be high.

7) Viscosity, Vapor pr., Freezing point. Should be low for ease in

handling & storage. Solvent should be nontoxic,nonflammable

& low cost.

Types :


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1) Stage-wise contact

a) Multistage crosscurrent Extraction.

b) Continuous countercurrent Extraction with Reflux.

c) Countercurrent multistage.2) Differential (continuous contact extractor)

a) Multistage crosscurrent Extraction

Raffinate is successively contacted with fresh solvent .It can be continuous or in Batch.

F

E1 E2 E3

S1S2 S3

R1 R2 R3

b) C ti t t E t ti ith R fl


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b) Continuous countercurrent Extraction with Reflux.

Solventseparator

E1 E2 E3 E4 S

R0

Extract

product

R1 R2 R3 R4

feed

F

Solvent

c) Countercurrent Multistage

F R1 R2 R3 R4

SE


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Single stage : MixerSettler

Mixer : For contacting two liquid phase to bring Mass transfer.

Settler : For mechanical separation.

Mixer : Series of orifices or mixing nozzle through which liquid to be

contacted are pumped co-currently.

Agitated vessel :For continuous operation liquid enters at bottom & leave at top.

In some cascade arrangement light & heavy liq. enters through side

wall near the top & bottom of the vessel respectively & leave through

The port in the wall opposite the impeller.

For batch operation mixing vessel itself acts as settler.

Impeller : Flat blade turbine type.

Dia. Of impeller / Tank dia. = 0.25 to 0.33

S ttl


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Light liq.

Heavy liq.

Dispersion

band.

Settler :

Decanter.

a) Sufficient residence time

b) Estimation of rate of flow to produce suitable dispersion band

thickness.

c) Calculation of time to settle individual drop through clear liq.above & below dispersion band.

Multistage countercurrent :

Recommended for systemsof low interfacial tension.

Light liquid out

Heavy liquid in

Light liquid in

Heavy liquid out

Differential continuous contact Extractor :


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Countercurrent flow is produced by difference in density of liquid.

1) Spray towers

2) Packed towers.

Mechanically agitated extractor :

For systems of high interfacial tension where density difference are

likely to be 1/10th as large or less,good dispersion of system of high

interfacial tension is impossible & mass transfer rates are poor.For such

systems dispersion is brought about by mechanical agitation of liquid.

Packed Towers :

Packing reduces axial mixing.Void space is filled with continuous

heavy liquid which flows down.Remainder of the void space is filledwith droplets of light liquid. Packing should be sufficiently small

& not greater than 1/8th of tower dia.

If the dispersed liquid wets the packing ,it will pass through as rivulets

& not as droplet & interfacial area produced will be small , for this

reason,packing material should be wetted by continuous phase.


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Economic Balance :

Amount of solute extracted for a fixed solvent/feed ratio increases

with increased no. of stages.

For a fixed extent of extraction ,the no. of stages required decreasesas solvent or reflux ratio increases.

Total cost ,sum of investment & operating cost must pass through a

min.,at a optimum solvent rate or Reflux ratio.

As solvent rate increases ,cost of solvent removal increases.

Correlation for estimating flooding rates ,axial mixing ,Mass transfer

rates are available.


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ETHERMAX PROCESS

PROCESS FLOW

OLEFINFEED

FRESH

WATER

PIC

LR

LI

TOSTATICMIXER

LPSteam

Condensate

TRC

FRC

LIC

WASTE WATERTO COKING UNIT

CoolingWater

LIC

FRC

ETHERMAXRAFFINATE

RAFFINATE TOHYDROTREATING

OLEFIN WASH COLUMN RECYCLE WASH COLUMN OLEFIN FEED SURGE DRUM

DEGASSING DRUM


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A D S O RP T I ON


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Topics covered.

What is Adsorption.

Different terms & its technical significance.

Physical & Chemical Adsorption.

Adsorption Isotherm.

Mass transfer characteristics, Equations.

Industrial application.


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ADSORPTION

Adsorption : Tendency of a molecule from a fluid / gas phase

to adhere to the surface of the solid.

The molecule which adsorbs is called as Adsorbate & the

surface on which it adsorbs is called Adsorbent.Separation occurs ,due to difference in MW,shape or polarity

which causes some molecules to be held strongly on the surface

than others.

For gas phase adsorption force field creates a regime of low PEnear the solid surface,molecular density near the solid surface is

generally > bulk gas density.

Rate of Adsorption from liquid is much slower than from gas.


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Adsorbent : Selectivity

Capacity

Mass transfer rateLong term stability.

Equilibrium capacity : How much of adsorbate will beadsorbed under given system.

Adsorption rate : How fast the adsorbate be adsorbed

under these condition.

Life : How many times the operation repeated.


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Adsorptive properties : Pore size / Pore size distribution

Nature of solid surface.

Size of bed : Gas / Liquid flow rate.

Desired cycle time.

Superficial velocity is usually 0.150.45 m/s.

Height of bed : Pressure drop.

Depending on the adsorbent the distribution of the pore dia.

within the adsorbent particle may be narrow ( 20 to 50 0A)or it may range widely ( 20 to several thousand 0A)


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Downward flow allows use of higher velocity.

If upward flow is selected it should be less than that atWhich lifting of Adsorbent occurs.

Fluidisation or violent agitation leads Adsorbent attrition &

loss of Adsorbent,higher pressure drop.

To avoid distribution & channeling use proper L/D ratio.

Use proper device to distribute flow uniformly. Avoid local

high velocity & eliminate particle movement & channeling.

Adequate bed support should be provided.


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Short cycle time : 1) Loss of operating flexibility.

2) Life of Adsorbent shortened.

3) High operating cost.4) Less efficient use of Total Adsorbent.


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Adsorbent must have high specific area & highly porous

structure with micropores.

Adsorbent surface adsorbs different components with

different affinities.

Depending on the nature of surface forces the adsorptionis of two types : 1) Physical Adsorption.

2) Chemisorption.

Adsorption process types : Continuous / Batch

Efficiency of Adsorption process is higher in continuous

mode of operation than in Batch mode of operation.


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Physical Adsorption Chemical Adsorption

Forces Weak Van der walls Strong forces,Electron

transfer,Bond formed

betn adsorbate & surface.

Heat of < 2 or 3 times latent > 2 or 3 times latentAdsorption heat of evaporation. heat of evaporation

Nature of Monolayer/multilayer

Adsorbed No dissociation of

phase adsorbed species.

Specificity Non specific Highly specific.

Monolayer.

Dissociation of

Adsorbed species.


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Physical Adsorption Chemical Adsorption

Reversibility rapid, non-activated activated,may be slow

reversible & irreversible.

Temp. range Significant at relatively Possible over a wide

low temp. Range of temp.


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Selectivity in Physical Adsorption

Majority is based on Equilibrium based selectivity.

Kinetic selectivity is restricted to molecular sieve adsorbent.

Normally down-flow is preferred as,up-flow at high rates

might fluidise the particles,causing attrition & loss of fines.

Equilibrium

Kinetics.

Adsorption is Exothermic process. Heat of adsorption

depends on loading.

A h


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Crystalline adsorbent : e.g Zeolite,alumna phosphate

No distribution of pore size.

Bed of adsorbent is supported on screen or perforated plate.

High area : lacks physical strength. & hence limits the

application.

Regeneration --- Hot inert gas is used.

Amorphous adsorbent : Silica , Alumna , activated carbon

Specific area : 2001000 m2/g

max. 1500 m2/g

AdsorbentCrystalline.

Amorphous

Adsorption Isotherm : Equilibrium relationship between


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For gas concentration is expressed in mole% or partial pr.

For Liquid phase concentration is expressed in ppm.

All system show a decrease in the amount adsorbed with an

increase in temperature.Working capacity of adsorbentdepends on fluid conc. & temperature.

Adsorption Isotherms :Linear

Concave upward

Convex upward.

Adsorption Isotherm : Equilibrium relationship between

Concentration in the fluid phase & the concentration in

the adsorbent particles at given temp.

Extent of adsorption is greater ,the smaller the solubility in

Solvent.


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W

g adsorbed

g solid

C ppm

Equilibrium capacity is determined from the adsorption

isotherm.

Conc. of adsorbate on solid Mass adsorbed per unit mass

of original adsorbent.

I ibl d i V f bl


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Irreversible adsorption : Very favourable

Amount adsorbed is independent of conc. down

to very low value.

Linear isotherm : Amount adsorbed is directly proportional to

concentration in fluid phase..

Concave upward isotherm : Unfavourable.Relatively low solid loadings are obtained.

It leads to long Mass transfer Zone in the bed.

Convex upward : Favourable

Relatively high solid loading obtained at low

conc in fluid.


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Langmure Isotherm:W = Wmax ( Kc / (Kc + 1) )

W = Adsorbate loading

K = Adsorption constant.

C = Conc in the fluid.

Freundlich Equation :

W = b cm

b & m are constants. For liq. M < 1.0


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t1t2

t3 t4

LBed length

C / C0

C / C0

time

tb


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Break point time Capacity of solid

Break point time 1/ Feed conc.

Mechanism of transfer to solid includes diffusion through

the fluid film around the particle & diffusion through the

pores to internal adsorption sites.Actual process of physical

adsorption is instantaneous & equilibrium is assumed to

exists between surface & the fluid at each point inside the

particle .

For mass transfer to take place the conc.in the fluid phase

in equilibrium with the solid phase should be less than theactual fluid conc.


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Where the conc. Profile is steep the difference in conc. is

large & mass transfer is rapid.

Width of mass transfer zone depends on the mass transfer rate,

flow rate & the shape of the equilibrium curve.

Narrow Mass Transfer Zone : Efficient use of adsorbent

Energy cost of regeneration is low.

Performance of Adsorbent bed is predicted from equilibrium

data & mass transfer calculation.

Usually adsorption are scaled up from lab tests in small dia.


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y p p

bed & large unit is designed for the same particle size &

superficial velocity.

p (1- ) dW

dt = Kc a (CC*)

(1- )p

= b = Bed density.

Kc = Overall mass transfer coefficient.

Kc int

& Kc ext

Kc

depends on

: provides insight into the mechanism by which adsorption

occurs.Difficult to determine & tedious to use.K

c

Higher the conc.of solute ,higher the equilibrium adsobate

conc.on the adsorbent.


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Diffusion within the particle is unsteady state process.

Kcint

=10 De

Dp

De = Effective diffusivity

Dp = Dia. Of particle.

Effective diffusion coefficient depends on particle porosity,

pore dia. & nature of diffusing species.

For adsorption of solute from aq. Solution internal diffusion

resistance often determine the transfer process & surface

migration is much less important.


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1) External film resistance.

2) Intraparticle diffusional :

Macropore diffusional

resistance

Micropore diffusional

resistance

Depending on the particulate system any one of these

resistance may be dominant or the overall rate of mass

transfer is determined by the combined effect of more

than one resistance.

True driving force for diffusion transport is gradient of

chemical potential rather than conc. difference.


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Usually mass transfer is governed by pore diffusion inside

the particle.

The size range of particle should be narrow.

Largest particle control the rate & gives lowest adsorptionperformance.Smallest particle controls pressure drop.

Particle size of Adsorbent affects

1) Mass transfer rate.

2) Pressure drop.

3) Maximum lifting velocity.


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Adsorption is always Exothermic.

From 1st & 2nd law of Thermodynamics.

F = H ST

All adsorption process proceeds spontaneously hence

F is always - ve.

S isve since system prior to adsorption exists in less

orderly state.

Hence H will always be negative.


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De-sorption require much higher temp. when adsorption is

strongly favourable or irreversible than when isotherms areLinear.

Process specific concerns:

Adsorbent age.

Loosing capacity because of fouling.

Loss of surface area or crystallinity. Oxidation.

Mass transfer resistance increase over time.


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UOP adsorbent used in PP plant for removal of sulfur species

Design conditions :

Feed rate : 100 MT/hr. , Temperature = 40 deg C , pressure= 22 bar a & composition 100% propylene.

AZ300 is better for heavier Sulfur species.

The adsorbent charged in C 2018 :SG 731 : 26 TAZ 300 : 12 T

UOP has suggested us to charge SG731 : AZ300 in theratio 2.26 : 1.0 .


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The worst case referred to UOP was as follows.

At inlet

H2S 2.0 ppm max.Mercaptan 1 ppm ( max)DMS < / = 0.1 ppmCOS < / = 0.1 ppmOthers < / = 0.1 ppm( Others : DMDS/Thiopene/CS2/higher mercaptans )

At outlet :

Total Sulfur : 0.1 ppm

As per UOP , for worst case of S impurities ,the bed will be duefor regeneration after 12 days.This indicates that the adsorbentbed has capacity of removing 83.52 kg of total sulfur species.


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T H AN K Y O U

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Mass Transfer Basics

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