1 (7)effect of pressure when total pressure p increases, or temperature t decreases, the solubility...

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(7)Effect of pressure (7)Effect of pressure

•When total pressure P increases, or temperature T decrWhen total pressure P increases, or temperature T decreases, the solubility of soluble components increases. eases, the solubility of soluble components increases. IncIncreasing P and decreasing T are Favorable conditions for reasing P and decreasing T are Favorable conditions for absorption.absorption. 吸收吸收 ( ( 高压、低温有利高压、低温有利 ) )

•If the If the gas and liquid rates are kept constantgas and liquid rates are kept constant so the oper so the operating line is not changed, for given ating line is not changed, for given ZZTT=H=Hoyoy••NNoyoy =Constant=Constant,,

•For given V, yb, ya, PFor given V, yb, ya, P x* x*bb=y=ybb/m/m

liquid flow rates Lminliquid flow rates Lmin..

ab

ab

xmy

yyVL

)(

min

baA xoryNforceDrivingmP

L

aboy y

yyN

)(

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(8)Temperature variations in packed towers(8)Temperature variations in packed towers

•When When rich gasrich gas is fed to an absorption tower, the temper is fed to an absorption tower, the temperature in the tower varies appreciably from bottom to top.ature in the tower varies appreciably from bottom to top.

•Heat of absorption of the solute Heat of absorption of the solute temperature temperature TT;;

•Evaporation of the solvent Evaporation of the solvent temperature temperature TT;;

•Usually, overall effect: Usually, overall effect: TT.. TTp*(equilibrium partial pp*(equilibrium partial pressure)ressure) Driving forceDriving force. The heat of absorption of the . The heat of absorption of the solute is disadvantageous to absorption solute is disadvantageous to absorption Factors influenciFactors influencing the temperature profile:ng the temperature profile: 1)rates of solute absorption; 1)rates of solute absorption; 2)evaporation or condensation of solvent; 3)heat transfe2)evaporation or condensation of solvent; 3)heat transfer between the phases.r between the phases.

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•When the gas inlet temperature is close to the exit When the gas inlet temperature is close to the exit temperature of the liquid and the incoming gas is temperature of the liquid and the incoming gas is saturated, there is little effect on solvent evaporation, saturated, there is little effect on solvent evaporation, and the rise in liquid temperature is roughly and the rise in liquid temperature is roughly proportional to the amount of solute absorbed. proportional to the amount of solute absorbed. [[Fig.18.15a, p.567Fig.18.15a, p.567]]

Tx•When the gas enters the columns 10 to 20 When the gas enters the columns 10 to 20 00C below C below the exit liquid temperature and the solvent is volatile, the exit liquid temperature and the solvent is volatile, evaporation will cool the liquidevaporation will cool the liquid in the bottom part of in the bottom part of the column, and the temperature profile may have a the column, and the temperature profile may have a maximum, as shown in maximum, as shown in Figure 18.15b.Figure 18.15b.

•*E.g.(18.4)*E.g.(18.4)

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(9)Supplementary: Stripping factor method for (9)Supplementary: Stripping factor method for calculating the number of transfer unitscalculating the number of transfer units

•Assume Assume

Operating-line equation:Operating-line equation:

)6.18(V

xLyVx

V

Ly aaaa

That is:That is:

)()( byyL

Vxx aa aa x

V

Lyx

V

Ly

5

)()( byyL

Vxx aa

Substituting Substituting xx from Eq.(b) into Eq.(a) and rearrange from Eq.(b) into Eq.(a) and rearrangement givement give

,, bmxySL

mVLet aa IntegrationIntegration(( 综合综合 )) of above E of above E

q. givesq. gives

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Syy

yyS

SN

aa

aboy )1(ln

1

1

SmV

LA

VL

m

L

mVS

1

)/(

Slope of equilibrium lineSlope of equilibrium line

Slope of operating lineSlope of operating line

]143,115.[,

Figurepyy

yy

L

mVfN

aa

aboy

[Similar to Eq.(3-84) in Chinese textbook] 脱吸因数法

脱吸因数

7

Syy

yyS

SN

aa

aboy )1(ln

1

1

]143,115.[,

Figurepyy

yy

L

mVfN

aa

aboy

Ln

VnOGoy

aba

q

mqSSNN

mxyyyyy

,

,

212

;

;;;

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Absorption efficiency Absorption efficiency AA::

)1( Abab

abA yyor

y

yy

S

y

yS

SN

a

boy )1(ln

1

1

WhenWhen 00, aa yxmxy

SS

SN

Aoy 1

1)1(ln

1

1

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•Two ways of increasing absorption efficiency Two ways of increasing absorption efficiency AA::

•1)Increasing the mass transfer driving force: (a)Increa1)Increasing the mass transfer driving force: (a)Increasing the sing the liquid-gas ratioliquid-gas ratio 液气比 L/V L/V driving forcedriving force; ; (b)Improving the equilibrium relationship: (b)Improving the equilibrium relationship: PP or or TT ddriving forceriving force; (c)x; (c)xaa driving forcedriving force..

•2)Decreasing the resistance to mass transfer (1/Kya)2)Decreasing the resistance to mass transfer (1/Kya): : (a)V (a)V (Gas-film controls);L(Gas-film controls);L (liquid-film controls). (liquid-film controls).

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•Noy Noy or or NoxNox reflects the reflects the difficultydifficulty of absorption proc of absorption process. The greater the value of Noy is, the more difficuless. The greater the value of Noy is, the more difficulty is the absorption process. ty is the absorption process.

•HoyHoy or or HoxHox reflects the reflects the performanceperformance of the absorpti of the absorption equipment.on equipment.

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•(10)Desorption or stripping(pp.81~85) (10)Desorption or stripping(pp.81~85) 解吸解吸•1)Definition of1)Definition of Desorption/strippingDesorption/stripping: the reverse of gas : the reverse of gas absorption: recover valuable solute from the absorbing absorption: recover valuable solute from the absorbing solution and regenerate the solventssolution and regenerate the solvents ..

•In stripping, the transfer is from the liquid to the gas pIn stripping, the transfer is from the liquid to the gas phase.hase.

•2)Characteristics (comparing with absorption): 2)Characteristics (comparing with absorption):

•Identical pointsIdentical points: (a)The : (a)The equipmentequipment used is the used is the samesame as as in absorption(plate or packed tower); (b)in absorption(plate or packed tower); (b)PrinciplesPrinciples are are the the samesame, both are of one way diffusion; (c), both are of one way diffusion; (c)Methods anMethods and equations for calculating Zd equations for calculating ZTT(height of a stripping colu(height of a stripping colu

mn)mn) are the are the samesame as for absorption. as for absorption.

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•Different points:Different points:

•(a)Purpose of absorption:(a)Purpose of absorption: Separating components froSeparating components from gas mixtures;m gas mixtures; Purpose of stripping:Purpose of stripping: Recover the solu Recover the solute and regenerate the solvents.te and regenerate the solvents.

•(b)Mass transfer direction:(b)Mass transfer direction: GasGasliquid (in absorptioliquid (in absorption);n); Liquid Liquidgas (in stripping).gas (in stripping).

•(c)Mass transfer driving forces:(c)Mass transfer driving forces: y>y*,(y-y*)in absorptiy>y*,(y-y*)in absorption;on; y<y*,(y*-y)in stripping. y<y*,(y*-y)in stripping.

•(d)Favorable conditions: (d)Favorable conditions: PP and T and T for absorption; for absorption; PP and Tand T for stripping. for stripping.

•(e)Positions of equilibrium line and operating line: see (e)Positions of equilibrium line and operating line: see the following figures.the following figures.

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Driving force:Driving force:

y – y*y – y*

Operating lineOperating line

Equilibrium curveEquilibrium curve

Equilibrium And Operating Lines for gas Equilibrium And Operating Lines for gas absorptionabsorption

y*y*

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For strippingFor stripping

Driving force:Driving force:

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•Calculations for strippingCalculations for stripping

•Stripping mediaStripping media: Inert gas or superheated steam(condens: Inert gas or superheated steam(condensable).able).

•1)Molal flow rate of Inert gas or superheated steam V=?1)Molal flow rate of Inert gas or superheated steam V=?

Higher conc. endHigher conc. end

Lower conc. endLower conc. end•Figure. packed Figure. packed column for stripping.column for stripping.

Inert gas or steamInert gas or steam

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•Comparing absorption: Comparing absorption:

•By balancing the operating costs against the fixed By balancing the operating costs against the fixed costs of the equipment,costs of the equipment,

min)/)(5.1~1.1()/( VLVL opt

•That That is, is,

min)5.1~1.1( LL

•For stripping,For stripping, min)5.1~1.1( VV

•Vmin is determined by calculaVmin is determined by calculating (V/L)min, i.e., (L/V)max:ting (V/L)min, i.e., (L/V)max:

ba

ba

xx

yy

V

L

max,

max



V

Lslope

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ba

ba

xx

yy

V

L

V

L

max,

minmax

•For case (a),For case (a), aa yy max,



V

Lslope

(a)(a)



V

Lslope

(b)(b)

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•2)Calculating Z2)Calculating ZTT(height of a stripping column) (height of a stripping column)

[Equations are the same as for absorption][Equations are the same as for absorption]

Gas film:Gas film:

Liquid film:Liquid film:

Overall gas:Overall gas:

Overall liquid:Overall liquid: )24.18(/

)23.18(/

)22.18(/

)21.18(/

b

a

b

a

b

a

b

a

x

x

oxx

ox

y

y

oyy

oy

x

x ix

xx

y

y iy

yy

oxoxoyoyxxyyT

xx

dxN

aK

SLH

yy

dyN

aK

SVH

xx

dxN

ak

SLH

yy

dyN

ak

SVH

NHNHNHNHZ

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aa

bb

aabbL

yyyy

yyyyy

ln

)()(

aa

bb

aabbL

xxxx

xxxxx

ln

)()(

Or,Or,

Syy

yyS

SN

aa

aboy )1(ln

1

1

20

[Example 18.5.][Example 18.5.]

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•1)Multicomponent1)Multicomponent （多组分）（多组分） absorption(pp.81)absorption(pp.81)

•Definition:Definition: Absorption process in whichAbsorption process in which more than onmore than one solute is absorbed by a solvent from a gas mixture.e solute is absorbed by a solvent from a gas mixture.

•Characteristics: Characteristics: Separate equilibrium and operating liSeparate equilibrium and operating lines are needed for each solute, but the slope of the opernes are needed for each solute, but the slope of the operating line, which is ating line, which is L/VL/V, is the same for all the solutes. , is the same for all the solutes.

•*Other absorption processes*Other absorption processes （自学）（自学）

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•Usually the packed height ZUsually the packed height ZTT and the liquid and the liquid

flow rate L are determined according to the flow rate L are determined according to the absorption requirement of some key component absorption requirement of some key component A, then the fractional removal of other A, then the fractional removal of other components are calculated from the given Zcomponents are calculated from the given ZTT

and L.and L.

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•2)Absorption from rich gases2)Absorption from rich gases (pp.574~580) (pp.574~580) （自学）（自学）•Solute concentration is moderate or high in the gas.Solute concentration is moderate or high in the gas.

•(a) (a) VV and and LL must be accounted for in the material b must be accounted for in the material balance; (b)The correction factor for one-way diffusion alance; (b)The correction factor for one-way diffusion should be included; (c)should be included; (c)KKyy or or KKxx will not be constant; (d) will not be constant; (d)

There may be an appreciable temperature gradient in There may be an appreciable temperature gradient in the column, which will change the equilibrium line.the column, which will change the equilibrium line.

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•3)Absorption with chemical reaction (pp.3)Absorption with chemical reaction (pp.588~589)588~589) （自学）（自学） •Absorption followed by reaction in the liquid phAbsorption followed by reaction in the liquid phase is often used to get more complete removal of ase is often used to get more complete removal of a solute from a gas mixture.a solute from a gas mixture.

•Characteristics: (a)Equilibrium partial pressurCharacteristics: (a)Equilibrium partial pressure of the solute p*e of the solute p* and and p*p*00; Driving force (p-p; Driving force (p-p*)*) . (b)Liquid film resistance to mass transfer. (b)Liquid film resistance to mass transfer , that is , that is kkLL (gas film controls)(gas film controls). (c)The . (c)The solvent isolvent i

s hardly saturateds hardly saturated by the solute by the solute.. Therefore, abso Therefore, absorption with chemical reaction can be used to subrption with chemical reaction can be used to substitute some difficult physical absorption processstitute some difficult physical absorption processes. es.

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•*[Mass transfer correlations*[Mass transfer correlations

•Empirical correlations for mass transfer coefficiEmpirical correlations for mass transfer coefficients in absorptionents in absorption

•1)For NH1)For NH33 absorption in water: absorption in water:

)782(1007.6 39.09.04 WGakg

•The absorption of The absorption of NHNH33 in water in water is often cited as is often cited as

an example of an example of gas-film controlgas-film control, since , since the gas film the gas film has about 80 to 90 percent of the total resistance.has about 80 to 90 percent of the total resistance.

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•2)For CO2)For CO22 absorption in water at 1 atm: absorption in water at 1 atm:

)792(57.2 96.0 UakL

•Liquid film controls: KLiquid film controls: KLL kkLL..

•3)For SO3)For SO22 absorption in water: absorption in water:

•With gases of intermediate solubility both resistWith gases of intermediate solubility both resistances are important.ances are important.

)802(1081.9 25.07.04 WGakg

)812(82.0 aWakL

1 (7)effect of pressure when total pressure p increases, or temperature t decreases, the solubility...

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