mass transfer - absorption

7/25/2019 Mass Transfer - Absorption

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Absorption


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AbsorptionAbsorption:-

It is a gas-liquid mass transfer operation in which liquid solvent is

contacted with gas mixture for differential dissolution of one or more

components of gas and provide a solution of them in liquid.

Uses of absorption:-

1- Purification of gases (H S from H!s".

#-Separation of gases (separation of dr$ gas % & ' from )* % & '.

+- Production of useful liquid product:- H (g" , H (liq" H (liq" #) (g" , H (liq" H) , H) # S (g" , H (liq" H S


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Physical vs chemical absorption:-

There are 2 types of absorption processes: physical

absorption and chemical absorption, depending onwhether there is any chemical reaction between thesolute and the solvent (absorbent.!hen water and hydrocarbon oils are used asabsorbents, no signi"cant chemical reactions occurbetween the absorbent and the solute, and theprocess is commonly referred to as physicalabsorption.

!hen a#ueous sodium hydro$ide (a strong base isused as the absorbent to dissolve an acid gas,

absorption is accompanied by a rapid andirreversible neutrali%ation reaction in the li#uidphase and the process is referred to as chemicalabsorption or reactive absorption.


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&ore comple$ e$amples of chemical absorption areprocesses for absorbing '2and )2* with

a#ueous solution of mono +ethanolamine (&-,

di +ethanolamine (.-, di+ethyleneglycol (./or tri+ethyleneglycol (T/, where a reversiblechemical reaction ta0es place in the li#uid phase1'hemical reactions can increase:-

1-the rate of absorption.#-increase the absorption capacity of the solvent.

+-increase selectivity to a certain components of thegas, and convert a ha%ardous chemical to a safe

compound.


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hysical absorption:-


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'hemical absorption:-


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Applications of absorption:

H$drogen sulfide(H#S" is removed from h$drocar/on gases /$ washing withal0aline solution (mines".

2ashing ethanol vapors from car/on dioxide from molasses fermentor tan0s

with water to remove ethanol.

cetone can /e recovered from acetone-air mixture /$ passing the gas streaminto water in which acetone is dissolved while air is passed out.

ar/on dioxide present in air is a/sor/ed /$ sodium h$droxide ()aH

solution" in which chemical a/sorption ta0es place.

)itrogen oxides are a/sor/ed in water to give nitric acid.

3emoval of ammonia coming from co0e ovens /$ water


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Choice Of Solvent For Gas Absorption

The factors to be considered are

High absorption power

!hich means that gas solubility should be high inthe solvent, which results in increasing the rate of

absorption and decreasing the #uantity of solventre#uired.

Highly Selective

The selectivity of solvent must be high in whichsolvent dissolve one and leave the others.

Easy to recover

!hich means easily to be regenerated.


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Low volatility

4he solvent should have a low vapor pressure to reduce loss of

solvent in the gas leaving the a/sorption column.

Small viscosityow viscosit$ is preferred for reasons of rapid a/sorption rates&

improving flooding characteristics in pac0ed column& low pressure

drops on pumping and good heat transfer characteristics.

Cost4he solvent should /e inexpensive& so that losses are not costl$& and

should /e readil$ availa/le.

Other properties)on-toxic& )on-flamma/le& )on-corrosive& hemicall$ sta/le& low

free5ing point


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Absorption E!ipments

"A# Plate Towers$%

3+&ultistage contact1

2+)igh separation , high capacity1

4+5elatively large diameter1

6+'ooling is done by

providing the platewith cooling coils1

7+ )igh pressure drop1

8+ asy to be clean1


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" Pac'ed Col!mns

3+.i9erential contact1

2+sed for highlycorrosive materials1

4+ *mall diameters ;= cm

6+?ot easy to clean1

7+ac0ing materials aremade from(ceramics ,

bric0s, wood, gravels,

stones , steel ,@@


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6-To increase surface area of contact between thetwo phases in pac0ed columns, ma0e more thanone section which increase the performance of

the tower.

7-cooling is done by dividing

the column

To many sectionsout side the column

(as seen in the opposite

Aigure".


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"C# Spray Col!mn$%

3+ 'ontinuous contact1

2+ Bow pressure drop1

4+ Bow eCciency1

6+ Bow cost(empty1

7+ /as phase controlling1

8+ 'onsidered as one stage1


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)((Wetted wall Colmn:-

*ingle tube wetted wallcolumn used in labs for

measuring mass transfercoeCcient.


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E!ilibri!m )elations$%

&ass transfer between /DB depends

highly on the e#uilibrium

between /DB1 .i9erent gases andli#uids yield separate solubility

curves , which must be

determined e$perimentally for

each system1 Ef the e#uilibrium

pressure of a gas at a given

li#uid concentration is high, as

case (- in the opposite "gure,

the gas is said to be relatively

insoluble in li#uid , while if itslow, as for curve (F , the

solubility is said to be high1


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E*ect of temperat!re on thee!ilibri!m c!rve

E*ect of temperat!re on thee!ilibri!m c!rve:-

The solubility of any gas is

inGuenced by the temperature1 Efthe temperature of the systemat e#uilibrium is raised , thesolubility of a gas decreases 1 -sshown in the opposite "gure as

temperature increases for thesame solute (gas the solubilitydecreases from (3=+8=o' andthe absorption power decreases.

-bsorption process is usuallyaccompanied by evolution ofheat1 *o Et is necessary to "tcoolers to the absorber to 0eepits temperature suCciently low.


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Types of E!ilibri!m )elations $%

Aor dilute concentrations of many gases thee#uilibrium relationship is given by )enryHs

law which relates the partial pressuredeveloped by a dissolved solute(- in ali#uid solvent (* by the following e#uation:+

-I ) $-

!here:+

)is )enryHs constant e$pressed as 0a Dmole fraction solute in li#uid,

-is the partial pressure of solute in 0a,

$-is the mole fraction of the gas in li#uid

phase


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)enryHs law holds very well whenthe partial pressure of the soluteis less than atmospheric1 -bove

atmospheric pressure , ) may beindependent of the partialpressure.

The variation of ) with temperatureis strongly nonlinear function.

Aor ideal systems 5aoultHs law isvalid:-

- I o- $-

!here

-, is the partial pressure of solute.

o-, is the vapor pressure of solute.

$- , is the mole fraction of the

solute in the li#uid phase.


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Fut if we increase theamount of li#uidsolvent ,the slope of

the operating line goesup and the drivingforce increases whichmeans small number

of stages isre#uired( also smallnumber of transferunits1 This means thatwe must ma0eoptimi%ation for li#uidamount as shown in

the opposite "gure.

!e have to use BD/ J

( BD/&in


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)&(Analytical method:-

Mremser e#uation represents ananalytical solution to a classical

separation problem of ? ideale#uilibrium stages concernedwith countercurrent gas andli#uid Gow1 The e#uilibrium andoperating relations are assumed

to be linear.Fy using the data of the 'eycomponentand by calculatingthe absorption factor"A+,-m.G# we can calculaten!mber of stagesfrom thefollowing chart1 -ftercalculating ? we can calculatethe recoveryfor each othercomponent.


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Stripping "desorption#:-

*tripping is the opposite ofabsorption and involves the

removal of dissolved gasesin li#uid by stripping agent.

urpose of stripping:-

1-recover the dissolved solute.

#-recover the solvent.

+-to recover both solute andsolvent.

/s!ally absorption isfollowed by stripping ordesorption.

The most commonly usedstripping agent is steam.


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Good stripping agent m!st be:-

1-easily condensed.

#-easily separated from the material stripped.

E!ilibri!m relations:-

(-s absorption"


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The following pointsm!st be ta'en into

consideration:-

1-perating line is underthe e#uilibrium curve.

#-slope of the operatingline

8BD/I (NHin+NHout D OHout+OHin

+--s (BD/ decreases, /increases , operating linegoes down, driving forceincreases, ? decreasesand ?T decreases.

9--s (BD/ increases, /

decreases, operating linegoes up, driving forcedecreases, ? increasesand ?T increases, tillwe reach pinch point.


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mass transfer - absorption

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