1 2. packed columns continuous contacting (differential contacting) equipment.continuous contacting...

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1 2. Packed Columns 2. Packed Columns Continuous contacting (differential Continuous contacting (differential contacting) equipment. contacting) equipment. Basic characteristics: Simple constr Basic characteristics: Simple constr uction; low pressure drop; packings e uction; low pressure drop; packings e asily made of corrosion asily made of corrosion 腐腐 腐腐 resistant m resistant m aterials. aterials. Applications: (1)Distillation with l Applications: (1)Distillation with l ow pressure drop and ow pressure drop and vacuum distillat vacuum distillat ion ion ; (2)Absorption and distillation w ; (2)Absorption and distillation w ith falling liquid flow rate far grea ith falling liquid flow rate far grea ter than upward vapor flow rate; (3)C ter than upward vapor flow rate; (3)C orrosive materials; (4)Small diameter orrosive materials; (4)Small diameter column; (5)Easily foaming materials; column; (5)Easily foaming materials; (6)Heat sensitive materials. (6)Heat sensitive materials.

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Page 1: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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2. Packed Columns2. Packed Columns•Continuous contacting (differential contacting) equContinuous contacting (differential contacting) equipment.ipment.

•Basic characteristics: Simple construction; low presBasic characteristics: Simple construction; low pressure drop; packings easily made of corrosionsure drop; packings easily made of corrosion 腐蚀腐蚀 resresistant materials. istant materials.

•Applications: (1)Distillation with low pressure drop Applications: (1)Distillation with low pressure drop and and vacuum distillationvacuum distillation; (2)Absorption and distillati; (2)Absorption and distillation with falling liquid flow rate far greater than upwon with falling liquid flow rate far greater than upward vapor flow rate; (3)Corrosive materials; (4)Smaard vapor flow rate; (3)Corrosive materials; (4)Small diameter column; (5)Easily foaming materials; (6)ll diameter column; (5)Easily foaming materials; (6)Heat sensitive materials.Heat sensitive materials.

Page 2: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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Illustrational Illustrational drawing of drawing of packed columnpacked column

Illustrational Illustrational drawing of drawing of packed columnpacked column

Page 3: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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Illustrational Illustrational drawing of drawing of packed packed columncolumn

Page 4: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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3-2-1 Packings3-2-1 Packings

1. Characteristics of packings 1. Characteristics of packings •Function of packings: Supplying space of mass tranFunction of packings: Supplying space of mass transfer. sfer. (1)Surface area per unit volume of packings (1)Surface area per unit volume of packings 比表面比表面积积 σ, σ, mm22 / /( m( m33 packing layer) packing layer) σ = n σσ = n σ0 0

σσ00=Surface area of a packing, m=Surface area of a packing, m22/a packing /a packing

n=Number of packings, n/mn=Number of packings, n/m33

Page 5: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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(2)Packing void fraction(2)Packing void fraction 空隙率空隙率 ε: ε: mm33 / /(m(m33 packing laye packing layer)r)

00 1 nV

V

NVV

V0 = Volume of a packing, mV0 = Volume of a packing, m33 /a packing /a packing (3)Packing factor(3)Packing factor 填料因子填料因子 : : σ/σ/ εε3 3 = Dry packing factor, = Dry packing factor, 1/m 1/m Φ=Φ=((σ/σ/ εε33))wetwet= Wet packing factor = Wet packing factor

(4)Stacking density (4)Stacking density ρρpp, kg/m, kg/m3 3

(Packing weight (Packing weight per unit volume of packings) per unit volume of packings)

[[Table 3-5Table 3-5][p.181]][p.181]

Page 6: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Basic requirement for packing: Large surface area Basic requirement for packing: Large surface area per unit volume of packing per unit volume of packing σσ; Large packing void fr; Large packing void fraction action εε; Low Stacking density ; Low Stacking density ρρpp; Good wetability; ; Good wetability;

Low fabricating cost; Enough mechanical strength. Low fabricating cost; Enough mechanical strength.

2. Types of packing2. Types of packing•Classification according to the structureClassification according to the structure(1)Solid packing: Rings/Saddles/Sheets (1)Solid packing: Rings/Saddles/Sheets (2)Gauze packing: Net-like saddles / (2)Gauze packing: Net-like saddles / θθ nets /Corruga nets /Corrugated netsted nets

Page 7: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Classification according to the manner of installation: Classification according to the manner of installation: (1)Random packings ;(1)Random packings ; (2)Structured packings. (2)Structured packings.

•Fluid mechanics and mass transfer performance compFluid mechanics and mass transfer performance comparison among several kinds of common used tower pacarison among several kinds of common used tower packings? (Rachig ring, Pall ring, Cascade ring [pp.178-18kings? (Rachig ring, Pall ring, Cascade ring [pp.178-181]1]

Page 8: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

8鲍尔环

阶梯环

Page 9: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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金属环矩鞍填料( Intalox Metal Tower Packing

球形填料

Page 10: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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波纹型规整填料 格栅型填料

Page 11: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Wall flow phenomenonWall flow phenomenon 壁流现象壁流现象 aand its decreasing.nd its decreasing.

Liquid redistributor of pacLiquid redistributor of packed columnked column

Page 12: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Main trends of new-type packing developmenMain trends of new-type packing development: t: (1)Increasing flux; (1)Increasing flux;

(2)Improving the uniform distributions of fluid flow; (2)Improving the uniform distributions of fluid flow;

(3)Decreasing flow resistance.(3)Decreasing flow resistance.

Page 13: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Two aspects of development: Two aspects of development: 1.Random packings: 1.Random packings: Rings and saddles. Rings and saddles. •Advantage of ring-type packings: Large flux; DiAdvantage of ring-type packings: Large flux; Disadvantage of ring-type packings: Bad liquid redistrisadvantage of ring-type packings: Bad liquid redistribution. bution. •Advantage of saddle-type packings: Good liquid rediAdvantage of saddle-type packings: Good liquid redistribution; Disadvantage of saddle-type packings: Smstribution; Disadvantage of saddle-type packings: Small flux. all flux.

•Combining the advantages of ring-type and saddle-tCombining the advantages of ring-type and saddle-type packings: Intalox saddle; Conjugate ring.ype packings: Intalox saddle; Conjugate ring.

Page 14: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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2.Structured packings: 2.Structured packings:

•Performance is much better than random packings Performance is much better than random packings with very low pressure drop, good liquid redistributwith very low pressure drop, good liquid redistribution, large flux. But the ion, large flux. But the fabricating cost fabricating cost is expensive.is expensive.

Page 15: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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3-2-2 Fluid Mechanics of packed 3-2-2 Fluid Mechanics of packed columnscolumns 填料塔流体力学性能填料塔流体力学性能

•Including: Including: (1)Pressure drop of gas flowing through pack(1)Pressure drop of gas flowing through packed sectionsed sections 气体压降气体压降 ; ; (2)Liquid flooding velocity(2)Liquid flooding velocity 液泛气速液泛气速 ; ; (3)Liquid and gas distributions(3)Liquid and gas distributions 气两相流体分布气两相流体分布 ; ;

(4)Liquid holdup(4)Liquid holdup 持液量持液量 ..

Page 16: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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1.Pressure drop of gas flowing through packin1.Pressure drop of gas flowing through packing depth (Relationship between g depth (Relationship between ΔΔP/Z~u)P/Z~u)

2

4 TD

Vu

Superficial gas velocity based on empty towerSuperficial gas velocity based on empty tower 空塔气速空塔气速u:u:

ⅠⅠ

ⅡⅡ

Gas velocity based on empty tower uGas velocity based on empty tower u

ⅢⅢ

Page 17: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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(1)L=0(1)L=0 :: Straight Straight line (Dry packing)line (Dry packing)

0.2~8.1uZP

(2)L (2)L 0: Curve 0: Curve •Two turning points: A(lTwo turning points: A(loading pointoading point 载点载点 ) and ) and B(flooding pointB(flooding point 泛点泛点 ). ). •Three zones: .ConstaⅠThree zones: .ConstaⅠnt liquid holdup zone; nt liquid holdup zone; ⅡⅡ.Liquid loading zone; .Liquid loading zone;

ⅢⅢ.Flooding zone..Flooding zone.

Logarithmic coordinatesLogarithmic coordinates

ⅠⅠ

ⅡⅡ

ⅢⅢ

Gas velocity based on empty tower uGas velocity based on empty tower u

Page 18: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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Question: Why Question: Why does the curve does the curve move left when move left when LL ??

•Under the same u, Under the same u, the the larger the Llarger the L is, is, the the more leftmore left the the curve is, that is, curve is, that is, LL (( P/ZP/Z ))

•Under the same V Under the same V and L and L 0 0 ,,Actual velocity uActual velocity uactualactual> Gas velocity of empty tower u> Gas velocity of empty tower u

Logarithmic coordinatesLogarithmic coordinates

ⅠⅠ

ⅡⅡ

ⅢⅢ

Gas velocity based on empty tower uGas velocity based on empty tower u

Page 19: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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uuactualactual> u, therefore, u> u, therefore, u

nder the same u, nder the same u,

((P/Z) with L P/Z) with L 0 is g 0 is greater than (reater than (P/Z) wiP/Z) with L=0. th L=0.

Logarithmic Logarithmic coordinatescoordinates

ⅠⅠ

ⅡⅡ

ⅢⅢ

Gas velocity based on empty tower uGas velocity based on empty tower u

Page 20: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Before point A, when Before point A, when uu, the liquid holdup of , the liquid holdup of the column does not the column does not change, this is change, this is constant constant liquid holdup zoneⅠliquid holdup zoneⅠ. . • After point A, when After point A, when uu, the liquid holdup of , the liquid holdup of the column increased, the column increased, this is this is liquid loading liquid loading zone Ⅱzone Ⅱ. .

•At point B and At point B and afterwards, flooding afterwards, flooding happens, this is happens, this is flooding flooding zone Ⅲzone Ⅲ..

Logarithmic coordinatesLogarithmic coordinates

ⅠⅠ

ⅡⅡ

ⅢⅢ

Gas velocity based on empty tower uGas velocity based on empty tower u

Page 21: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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Logarithmic coordinatesLogarithmic coordinates

ⅠⅠ

ⅡⅡ

ⅢⅢ

Gas velocity based on empty tower uGas velocity based on empty tower u

•Phenomena of floodinPhenomena of floodingg: The packed sections : The packed sections fluctuate violently fluctuate violently.. Pr Pressure drop of packed essure drop of packed sections rises more rapsections rises more rapidly idly ((PP )). Liquid ra. Liquid rapidly accumulates, anpidly accumulates, and the entire column md the entire column may fill with liquid. Flooay fill with liquid. Flooding is detrimental opding is detrimental operation. eration.

Page 22: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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2. Flooding 2. Flooding Normally, Normally, liquid liquid is the scattered phase and is the scattered phase and gasgas the c the continuous phase. ontinuous phase. When When flooding,flooding, liquidliquid becomes the becomes the continuous phasecontinuous phase and and gas the scattered phasegas the scattered phase. . •At flooding point B, the corresponding gas velocity At flooding point B, the corresponding gas velocity is uis uF F 泛点气速泛点气速

Page 23: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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(1)Factors influencing (velocity at flooding)(1)Factors influencing (velocity at flooding) 泛点气速泛点气速 uuF:F:

1) 1) Characteristics of packings:Characteristics of packings: 填料因子填料因子 uuFF (Fl (Fl

ooding will not happen easily). ooding will not happen easily). •For example, if For example, if void fraction void fraction is large enough, the perm is large enough, the permissible gas flux will be large enough too, and uissible gas flux will be large enough too, and uFF. . is a is a cc

omprehensive factoromprehensive factorcombining the influences of combining the influences of , , and and packing structures on column performance. packing structures on column performance. 2)Fluid physical properties: 2)Fluid physical properties: VV and and u uF F ; ; LL

uuFF . .

3)Ratio of liquid and gas flow rates3)Ratio of liquid and gas flow rates 液气比液气比 : :

(w(wLL/w/wVV)) u uFF

Page 24: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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(2)Determining the velocity at flooding u(2)Determining the velocity at flooding uFF (Eckert g (Eckert g

eneralized correlation: Figure 3-24, p.184)eneralized correlation: Figure 3-24, p.184)

2.022

1

L

V

L

V

V

L

g

uB

w

wA ,

lineFlooding

Page 25: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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(3)Determining (3)Determining (( P/ZP/Z ): ):

•After determining uAfter determining uFF, take u, take u =(0.5~0.85)u=(0.5~0.85)uFF ,, calculcalcul

ate A and Bate A and B ,, then determine then determine P/Z from Eckert genP/Z from Eckert generalized correlation.eralized correlation.

Page 26: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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3. Wettability3. Wettability•Wetted surfaces of packings are effective mass transfeWetted surfaces of packings are effective mass transfer area. r area. •Definition of Definition of spray density spray density U [p.185]: U [p.185]: Liquid volume sprayed per unit time per unit cross sectLiquid volume sprayed per unit time per unit cross section of the column, [mion of the column, [m33/m/m22.s] .s]

•Minimum spray density UMinimum spray density Uminmin: the lowest value of U tha: the lowest value of U tha

t ensures the packings wetted.t ensures the packings wetted.

Page 27: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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(L(LWW))minmin——Minimum wetting rate, [m——Minimum wetting rate, [m33/m.s] /m.s]

—— —— 填料的比表面积填料的比表面积

wetting rate: Liquid volume flow rate per unit length of wetting rate: Liquid volume flow rate per unit length of packing periphery.packing periphery.

(3-37)(3-37)

Page 28: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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3-2-3 Calculations of Packed Colu3-2-3 Calculations of Packed Columnsmns

1.Column diameter:1.Column diameter:

VVSS=gas volume flow rate, m=gas volume flow rate, m33/s /s

•Attention: (1) DAttention: (1) DTT should be rounded up; should be rounded up;

(2)Checking:(2)Checking:

(Avoiding wall flow)(Avoiding wall flow)

Page 29: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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2.Column height 2.Column height (1)Calculation from the number of transfer units: (1)Calculation from the number of transfer units: Z=HZ=HOyOy*N*NOyOy; etc. ; etc.

(2)Calculation from HETP: Z=HETP*N(2)Calculation from HETP: Z=HETP*NTT, etc., etc.

Page 30: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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3-2-4 Accessory S3-2-4 Accessory Structures of Packetructures of Packe

d columnsd columns

1.Packing supports1.Packing supports

(a)Grid plate(a)Grid plate

Page 31: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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•Function of packing support: Function of packing support: Supporting the packings and thSupporting the packings and the liquid holdup, ensure fluent fle liquid holdup, ensure fluent flow of gas and liquid. ow of gas and liquid. •Basic requirement: Basic requirement:

Enough mechanical strength, aEnough mechanical strength, and free cross sectional area.nd free cross sectional area.

(b)Riser(b)Riser 升气管升气管式式

Page 32: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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2.Liquid distributors 2.Liquid distributors

•Function: Uniformly distribute the liquid on the suFunction: Uniformly distribute the liquid on the surfaces of packings.rfaces of packings.

Shower nozzle typeShower nozzle type

Annular tubes with multi-holesAnnular tubes with multi-holesOverflow pipesOverflow pipes

Page 33: 1 2. Packed Columns Continuous contacting (differential contacting) equipment.Continuous contacting (differential contacting) equipment. Basic characteristics:

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3.Liquid redistributor 3.Liquid redistributor Function: Reducing the non-uniform distribution oFunction: Reducing the non-uniform distribution of liquid, and reducing the wall flow. f liquid, and reducing the wall flow. 4.Mist catcher (Entrainment eliminator)4.Mist catcher (Entrainment eliminator) Function: Eliminating the entrained liquid drops in Function: Eliminating the entrained liquid drops in the gas stream at the outlet. the gas stream at the outlet. 5.Gas inlet 5.Gas inlet Function: Ensuring the uniform distribution of gas Function: Ensuring the uniform distribution of gas flow on the cross sectional area of the column. flow on the cross sectional area of the column.

[Comparison between packed and plate towers?][Comparison between packed and plate towers?]