Chapter 11 Column Equipment

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1. Plate Columns1. Plate Columns•CascadesCascades (Fig.3-1)[p.147]: (Fig.3-1)[p.147]: •The column contains a number of pThe column contains a number of plates, or trays,stacked one above the lates, or trays,stacked one above the other. other.

•Compositions of liquid and gas phaCompositions of liquid and gas phases change ses change stepwisestepwise along the directi along the direction of column height.on of column height. 气液两相逆流气液两相逆流流动，每块板为错流流动。两相组流动，每块板为错流流动。两相组成变化沿塔高为梯级式变化。成变化沿塔高为梯级式变化。

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溢流堰

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Bubble-cap tBubble-cap trayray 泡罩塔泡罩塔

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Valve-traysValve-trays

F1 Valve-tray F-1F1 Valve-tray F-1 型型 V-4 Valve-trayV-4 Valve-tray

T-Valve-trayT-Valve-tray

浮阀浮阀

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•Types of plate columns: Types of plate columns: 11 ）） Trays Trays without weirswithout weirs 堰堰 : Penetrating flow/Coun: Penetrating flow/Countercurrent flowtercurrent flow 逆流式塔板逆流式塔板 [Less used][Less used]. .

22 ）） Trays with weirs: Cross flowTrays with weirs: Cross flow [widely used][widely used]: : •Bubbling traysBubbling trays: Sieve-tray: Sieve-tray 筛孔筛孔 columns; Bubble-columns; Bubble-cap tray columnscap tray columns 泡罩泡罩 ; Valve-tray columns; Valve-tray columns 浮阀浮阀 ; ; Inclined-opening tray columns. Inclined-opening tray columns.

•Jet traysJet trays 喷射型喷射型 : Tongued tray columns; Float jet : Tongued tray columns; Float jet tray columns; Floating tongued plate columns.tray columns; Floating tongued plate columns.

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•Tongued trayTongued tray 舌形孔舌形孔 将塔上冲压成斜向舌形孔，张角 20°左右，如图所示。气相从斜孔中喷射出来，一方面将液相分散成液滴和雾沫，增大了两相传质面，同时驱动液相减小液面落差。液相在流动方向上，多次被分散和凝聚，使表面不断更新，传质面湍动加剧，提高了传质效率。

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•Floating tongued plateFloating tongued plate 浮舌塔板若将舌形板做成可浮动舌片与塔板铰链，如图所示，称其为浮舌塔板，可进一步提高其操作弹性。特点：气液流动方向一致，充分利用气体动能来进行两相接触，提高分离效率，压降小，雾沫夹带小。

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•Approaches to and trends of improving performance Approaches to and trends of improving performance of plate columns: of plate columns: 1)Increasing the capacity; 2)Simplifying the structures;1)Increasing the capacity; 2)Simplifying the structures; 3)Decreasing the costs; 4)High efficiencies and low res 3)Decreasing the costs; 4)High efficiencies and low resistance; 5)Applying to large-size requirements. istance; 5)Applying to large-size requirements.

•Indexes for performance evaluation:Indexes for performance evaluation:•1)Production capacity; 2)Plate efficiencies; 3)Turndo1)Production capacity; 2)Plate efficiencies; 3)Turndown ratio wn ratio （操作弹性）（操作弹性） ; 4)Pressure drop of trays; 5)F; 4)Pressure drop of trays; 5)Fabricating cost.abricating cost.

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3-1-2 Fluid Mechanics of Plate Colum3-1-2 Fluid Mechanics of Plate Columnsns 塔板的流体力学性能塔板的流体力学性能

•Contact states of liquid-gas phases on trays: Contact states of liquid-gas phases on trays: 1)Bubbling :1)Bubbling : Low gas velocities; Small amount of bu Low gas velocities; Small amount of bubbles; Thick and clear liquid layer; Contacts betweebbles; Thick and clear liquid layer; Contacts between liquid and gas phases: Bubble surfaces; n liquid and gas phases: Bubble surfaces; Scattered Scattered phase: gas phasephase: gas phase. .

2)Foaming/Frothing: 2)Foaming/Frothing: Middle gas velocities; Highly tMiddle gas velocities; Highly turbulent froth; Little clear liquid layer; Contacts beurbulent froth; Little clear liquid layer; Contacts between liquid and gas phases: Frothtween liquid and gas phases: Froth 泡末泡末 surfaces (lisurfaces (liquid film); quid film); Scattered phase: gas phase.Scattered phase: gas phase.

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3)3) Jetting of liquid drops:Jetting of liquid drops: High gas velocities; Liquid becoming drops; High gas velocities; Liquid becoming drops; Contacts between liquid and gas phases: Surfaces of Contacts between liquid and gas phases: Surfaces of liquid drops; liquid drops; Scattered phase: liquid phaseScattered phase: liquid phase..

•Normal operation: Frothing orNormal operation: Frothing or Jetting of liquid Jetting of liquid drops.drops.

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（ a ） 泡 沫状

（ b ）喷射状

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(1). Vapor pressure drop(1). Vapor pressure dropΔΔp p （（ pp.163~165pp.163~165 ） ）

Total vapor pressure drop per plate: Total vapor pressure drop per plate: PPpp=P=Pn+1n+1-P-Pnn

ph

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（（ 3-13-199 ））

•The equivalent The equivalent heads of liquidheads of liquid in the column: in the column:

（（ 3-193-19aa ））

=total vapor pressure drop per plate, Pa; =total vapor pressure drop per plate, Pa; =friction loss for dry plate, Pa=friction loss for dry plate, Pa ； ； = vapor pressure drop overcoming liquid layer on = vapor pressure drop overcoming liquid layer on plate, Paplate, Pa ； ；

= vapor pressure drop overcoming liquid surface = vapor pressure drop overcoming liquid surface tension, Patension, Pa

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•The equivalent heads of liquid in the column:The equivalent heads of liquid in the column:

（（ 3-193-19aa ））

.,

;,

;,

;,

liquidofmg

ph

liquidofmg

ph

liquidofmg

ph

liquidofmg

ph

L

L

ll

L

cc

L

pp

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（（ 3-193-19aa ））Usually hUsually h very small and omitted.very small and omitted.

）（）（）（

223

]213203[

0

0

Ll

c

hh

andEqstoreferhu

PPppPressure of tower bottomPressure of tower bottom, but plate efficiency, but plate efficiency

Usually if plate efficiency is high enough, Usually if plate efficiency is high enough, PPpp must be kept as must be kept as

small as possible.small as possible.

=vapor velocity through holes, m/s =vapor velocity through holes, m/s =aeration factor, dimensionless =aeration factor, dimensionless

Under the same conditions, Under the same conditions, PPp,Bubble-capp,Bubble-cap>>PPp,valvep,valve > >PPp,sievep,sieve

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(2) Liquid Flooding (2) Liquid Flooding 液泛液泛 If If vapor or liquid flow rate is over certain limitvapor or liquid flow rate is over certain limit, vapor , vapor pressure droppressure drop between plates increases so as tobetween plates increases so as to hinder hinder the liquid flow in the downcomer, the liquid in the dothe liquid flow in the downcomer, the liquid in the downcomer backs up to the next plate; Or because wncomer backs up to the next plate; Or because the crthe cross section area of down-comer is too smalloss section area of down-comer is too small to let the li to let the liquid flow through the down-comer, the liquid in the dquid flow through the down-comer, the liquid in the downcomer backs up to the upper plate. owncomer backs up to the upper plate. Flooding is the upper limit of operation with countercFlooding is the upper limit of operation with countercurrent flow of liquid and vapor and must be avoided.urrent flow of liquid and vapor and must be avoided.汽液两相中之一的流量增大，使降液管内液体不能顺汽液两相中之一的流量增大，使降液管内液体不能顺利下流，管内液体积累，当管内液体增高到越过溢流利下流，管内液体积累，当管内液体增高到越过溢流堰顶部，两板间液体相连，塔板产生积液，并依次上堰顶部，两板间液体相连，塔板产生积液，并依次上升，造成液泛（淹塔）升，造成液泛（淹塔）

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=correct coefficient considering aeration and safe =correct coefficient considering aeration and safe operationoperation

Operating limit equation of Flooding is as follows:Operating limit equation of Flooding is as follows:

降液管内的清液层高度降液管内的清液层高度 HdHd

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Eq.(3-24) can be derived from Bernoulli equationEq.(3-24) can be derived from Bernoulli equation 伯努力方伯努力方

程程 . Between 1-1’ and 2-2’. Between 1-1’ and 2-2’ ：：

dL

nL

L

nd h

g

ph

g

pH

1

g

pph

L

nnp

1

hhdd=flow resistance from 1-=flow resistance from 1-

1’ to 2-2’, mainly in the do1’ to 2-2’, mainly in the down-comer.wn-comer.

HHdd=equivalent height of clear liquid.=equivalent height of clear liquid.

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•Factors influencing floodingFactors influencing flooding 液泛液泛 :: 1)Vapor and liquid flow rates;1)Vapor and liquid flow rates; 2)Physical properties of vapor and liquid; 2)Physical properties of vapor and liquid; 3)Plate structure; for example, distance between plate3)Plate structure; for example, distance between plates s HHTT, cross section area of down-comer , cross section area of down-comer AAff, spacing bet, spacing bet

ween downcomer bottom and tray ween downcomer bottom and tray hh00, etc., etc.

液速一定时，气速过大，液体不能顺利下流，管内液体积累，管内液体越液速一定时，气速过大，液体不能顺利下流，管内液体积累，管内液体越过溢流堰顶部，造成液泛。过溢流堰顶部，造成液泛。2 2 气速一定时，液体流量过大，降液管的截面不足以使液体通过，管内液气速一定时，液体流量过大，降液管的截面不足以使液体通过，管内液面上升，发生液泛。面上升，发生液泛。3 3 板的设计不好，或流体物性造成。（改变板间距，改造塔结构）。板的设计不好，或流体物性造成。（改变板间距，改造塔结构）。 44 当当气体上升夹带过量的液体，增加降液管的排液负荷，引起液泛。气体上升夹带过量的液体，增加降液管的排液负荷，引起液泛。 5 5 降液降液管堵塞管堵塞。。Question: How to avoid flooding?Question: How to avoid flooding?

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(3) Froth Entrainment (3) Froth Entrainment 雾沫夹带雾沫夹带The liquid drops on the tray is brought into the uppeThe liquid drops on the tray is brought into the upper plate and the plate enrichment is decreased.r plate and the plate enrichment is decreased.

Entrainment becomes excessive when the amount of Entrainment becomes excessive when the amount of entrainment eentrainment eVV is greater than 0.1 (e is greater than 0.1 (eVV>0.1). >0.1).

eeVV: kg of liquid/kg of vapor : kg of liquid/kg of vapor

Operating limit of froth entrainment is as follows: Operating limit of froth entrainment is as follows: eeVV0.1kg of liquid/kg of vapor 0.1kg of liquid/kg of vapor

It can also be determined by the It can also be determined by the percentage of floodipercentage of flooding point(percent flooding) ng point(percent flooding) [pp.165~166].[pp.165~166].

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Factors influencing froth entrainment: Factors influencing froth entrainment: From Eqs.(3-28), (3-29)[ p.166], for certain liquid-vapFrom Eqs.(3-28), (3-29)[ p.166], for certain liquid-vapor system, or system, the main factors are distance between platethe main factors are distance between plates s HHTT,, column diameter column diameter DD, vapor and liquid flow rates, , vapor and liquid flow rates,

VsVs and and LsLs, respectively, respectively. .

Question: How to avoid excessive froth entrainment? Question: How to avoid excessive froth entrainment?

雾沫夹带产生的原因：（两种），其一是板间距小。气相在液层中鼓泡，雾沫夹带产生的原因：（两种），其一是板间距小。气相在液层中鼓泡，气泡破裂，将雾沫弹溅至上一层塔板。增加板间距可减少夹带量。气泡破裂，将雾沫弹溅至上一层塔板。增加板间距可减少夹带量。

另一种原因是空塔气速高，气相运动是喷射状，将液体分散并可携带一部另一种原因是空塔气速高，气相运动是喷射状，将液体分散并可携带一部分液沫流动，此时增加板间距不会奏效。随气速增大，使塔板阻力增大，分液沫流动，此时增加板间距不会奏效。随气速增大，使塔板阻力增大，上层塔板上液层增厚，塔板液流不畅，液层迅速积累，以致充满整个空间，上层塔板上液层增厚，塔板液流不畅，液层迅速积累，以致充满整个空间，即液泛。即液泛。

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(4) Liquid Weeping (4) Liquid Weeping 漏液漏液 Liquid weeping happens when the vapor velocity (vapLiquid weeping happens when the vapor velocity (vapor kinetic energy) is too low to stop the liquid flowing tor kinetic energy) is too low to stop the liquid flowing through the vapor holes on the plate. The separation efhrough the vapor holes on the plate. The separation effect lowered.fect lowered.Operating limit of Liquid weepingOperating limit of Liquid weeping is as follows: is as follows:

The weeping amount should not be greater than 10% The weeping amount should not be greater than 10% of the liquid flow rateof the liquid flow rate,, or the weeping amount is deter or the weeping amount is determined by kinetic energy factormined by kinetic energy factor (F factor), e.g., for (F factor), e.g., for valvvalve tray, Fe tray, F0 0 5~65~6..

F is the index of dynamic pressure of vapor.F is the index of dynamic pressure of vapor.

Kinetic energy factorKinetic energy factor

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Factors causing weeping:Factors causing weeping:

1)1) vapor velocity too low; vapor velocity too low;

2)2) Nonuniform flow of vapor through a plate causNonuniform flow of vapor through a plate caused by ed by hydraulichydraulic gradient gradient [clear-liquid height diff[clear-liquid height difference]on plate. erence]on plate.

Kinetic energy factor of the openings in the valve-Kinetic energy factor of the openings in the valve-tray:tray:

=vapor velocity in the openings in valve-tray=vapor velocity in the openings in valve-tray

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In order to avoid weeping in liquid inlet with higher In order to avoid weeping in liquid inlet with higher liquid layer on plate, liquid layer on plate, a shakedown/defoaming a shakedown/defoaming （除（除泡）泡） areaarea is settled. is settled.

installation areainstallation area

Downcomer Downcomer areaarea

Active Active areaarea

Defoaming areaDefoaming area

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(5) Hydraulic gradient/clear-liquid height difference (5) Hydraulic gradient/clear-liquid height difference on plate: on plate:

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•Hydraulic gradient on plateHydraulic gradient on plate is caused by the liquid is caused by the liquid flow resistance across the plate: flow resistance across the plate: •1) friction loss through plate surface; 1) friction loss through plate surface; •2) local resistance of parts(bubble-cap, floating valve) on pl2) local resistance of parts(bubble-cap, floating valve) on plate; ate; •3) vapor flow resistance.3) vapor flow resistance. •The nonuniform liquid layer on plate will cause the The nonuniform liquid layer on plate will cause the nonuniform distribution of vapor flow, leading liquinonuniform distribution of vapor flow, leading liquid weeping .d weeping .

Main factors influencing weeping:Main factors influencing weeping:

1) plate structure; 2)column diameter; 3)liquid flow rate.1) plate structure; 2)column diameter; 3)liquid flow rate.

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•Question: How to reduce detrimental effects of hydrQuestion: How to reduce detrimental effects of hydraulic gradient/ clear-liquid height difference on plate? aulic gradient/ clear-liquid height difference on plate?

•1)Column diameter not too large; 1)Column diameter not too large;

•2)using multi-downcomers/split flow tray; 2)using multi-downcomers/split flow tray;

•3)using stepped plate construction/cascade cross flow 3)using stepped plate construction/cascade cross flow tray.tray.

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*Back mixing*Back mixing 返混： 返混： Because of vapor agitationBecause of vapor agitation 扰动扰动 , part of liquid on plat, part of liquid on plate may flow backward. Flow opposite to main flow dire may flow backward. Flow opposite to main flow direction is called ection is called back mixingback mixing. Back mixing lowers the . Back mixing lowers the plate efficiency. plate efficiency. *Vapor bubble entrainment*Vapor bubble entrainment 气泡夹带气泡夹带 : :

Velocity of Liquid overflowing into down-comer is toVelocity of Liquid overflowing into down-comer is too high to separate the vapor entrained by froth from o high to separate the vapor entrained by froth from liquid, and vapor is brought into the lower plate. liquid, and vapor is brought into the lower plate.

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Operating limit of vapor bubble entrainment: ResideOperating limit of vapor bubble entrainment: Residence time of liquid in downcomernce time of liquid in downcomer 降液管内液体停留降液管内液体停留时间时间 ::

sLHA Tf 5~3

max

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*Blowing*Blowing 干吹： 干吹： High vapor velocities, low liquid flow rate, the tray iHigh vapor velocities, low liquid flow rate, the tray is blown clear of liquid, and no effect of liquid seal. s blown clear of liquid, and no effect of liquid seal.

Operating limit of blowing:Operating limit of blowing:

mmhow 6

•The above mentioned The above mentioned liquid flooding, excessive liquid flooding, excessive froth entrainment, excessive weeping, vapor bubble froth entrainment, excessive weeping, vapor bubble entrainment and blowingentrainment and blowing are are 5 detrimental 5 detrimental operationsoperations and must be avoided. and must be avoided.

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(6) Capacity performance chart (6) Capacity performance chart 负荷性能图 负荷性能图 •For certain vapor-liquid system, there exists a zone of saFor certain vapor-liquid system, there exists a zone of satisfactory operation for vapor and liquid loadings within tisfactory operation for vapor and liquid loadings within a plate column. This is described by capacity performanca plate column. This is described by capacity performance charte chart

Five curvesFive curves correspond correspond to to 5 detrimental 5 detrimental operationsoperations, and are , and are plotted fromplotted from 5 operating 5 operating limit equations limit equations in ain a Vs—Vs—Ls diagram.Ls diagram.

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Curve 1: Curve 1: excessive froth entrainment line (straight);excessive froth entrainment line (straight); From Eq. (3-28)[p.166]:From Eq. (3-28)[p.166]: 雾沫夹带线雾沫夹带线 // 气相负荷上限气相负荷上限线线

Upper limit of percent floodingUpper limit of percent flooding

=Flooding load coefficient [Fig.3-16]=Flooding load coefficient [Fig.3-16]

),( TVF HfC =Liquid flow distance on tray, m=Liquid flow distance on tray, m

=Liquid flow area on tray, m2=Liquid flow area on tray, m2

=coefficient of physical =coefficient of physical properties(Table3-4)properties(Table3-4)

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Curve 2: Curve 2: liquid flooding curve. liquid flooding curve. From operating limit From operating limit equation of Flooding:equation of Flooding: 液泛线液泛线

dLlcdLpd

wTd

hhhhhhhhH

hHH

又)( (3-27) (3-27)

(3-24)(3-24)

Curve 3: Curve 3: Line of upper limit Line of upper limit of liquid flow rateof liquid flow rate (vertical li(vertical line).ne). From operating limit of From operating limit of vapor bubble entrainment, cvapor bubble entrainment, calculate(Lalculate(Lss))maxmax::

)(5~3)( max

sL

HA

s

Tf

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Curve 4: Curve 4: excessive weeping excessive weeping line (horizontal line). line (horizontal line). Operating limit of Liquid Operating limit of Liquid weeping:weeping:

Curve 5: Curve 5: Line of lower Line of lower limit of liquid flow ratelimit of liquid flow rate (vertical line).(vertical line). Operating Operating limit of blowing:limit of blowing:

is calculated from is calculated from Eqs.(3-6)~(3-8) for Eqs.(3-6)~(3-8) for calculating calculating hhowow..

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Line ab and line a’b’ are operating line. operating Line ab and line a’b’ are operating line. operating points P and P’ must fall into the satisfactory operpoints P and P’ must fall into the satisfactory operation zone.ation zone.

Turndown ratioTurndown ratio 操作弹性操作弹性 /Ope/Operating flexibility: Ratio of Vrating flexibility: Ratio of Vsasa

to Vto Vsbsb, , representing the chanrepresenting the chan

ging range of satisfactory opeging range of satisfactory operation.ration.

两极限的气体流量之比为塔板的两极限的气体流量之比为塔板的操作弹性操作弹性

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Use of capacity performance chart:Use of capacity performance chart:

1)Checking whether the design is satisfactory or not; 1)Checking whether the design is satisfactory or not;

2) Understanding the 2) Understanding the turndown ratioturndown ratio of the column; of the column;

3) Judging the potential of increasing capacity.3) Judging the potential of increasing capacity.

负荷性能图中被曲线所包围的区域，是设计的塔板分负荷性能图中被曲线所包围的区域，是设计的塔板分离给定物系的适宜操作范围，其区域越大，则适离给定物系的适宜操作范围，其区域越大，则适宜范围越大。只要设计点落在适宜操作范围内，宜范围越大。只要设计点落在适宜操作范围内，塔板即可正常运行。塔板即可正常运行。

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•If If operating point is near any of the 5 curvesoperating point is near any of the 5 curves, the , the design is considered as design is considered as not satisfactorynot satisfactory. In order to . In order to make the operating point fall into the middle of the make the operating point fall into the middle of the satisfactory operation zone, the tray structure and size satisfactory operation zone, the tray structure and size and the flow rates of vapor and liquid may be changed.and the flow rates of vapor and liquid may be changed.

Question: The operating point P’ Question: The operating point P’ of operating line a’b’ is near the of operating line a’b’ is near the line of upper limit of liquid flow line of upper limit of liquid flow rate. Are there any ways to make rate. Are there any ways to make the operating point fall into the the operating point fall into the middle of the satisfactory middle of the satisfactory operation zone?operation zone?

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•Increasing AIncreasing Aff, line of up, line of up

per limit of liquid flow raper limit of liquid flow rate will move to the right ate will move to the right and P’ will be near the midnd P’ will be near the middle. dle. •Any other ways to put P’ Any other ways to put P’ near the middle? near the middle?

• Can the line 4 get lower?Can the line 4 get lower?