1

(16) Use of (16) Use of Murphree efficiencyMurphree efficiency 默弗里板效默弗里板效率率 /Determining the number of /Determining the number of actual platesactual plates实际板实际板 [pp.692-693][pp.692-693]

•When the Murphree efficiency is known, it cWhen the Murphree efficiency is known, it can an readily readily be used in the McCabe-Thiele diagbe used in the McCabe-Thiele diagram. ram.

•Fig.21.33[p.693]Fig.21.33[p.693]

2

Fig.21.33Fig.21.33

1ny

1nx

3

•Triangle Triangle acdacd represents the ideal plate and t represents the ideal plate and triangle riangle abeabe the actual plate. the actual plate.

•The actual plate, instead of enriching the vaThe actual plate, instead of enriching the vapor from ypor from yn+1n+1 to y* to y*nn, accomplished a lesser en, accomplished a lesser en

richment yrichment ynn- y- yn+1n+1..

4

•To apply a known Murphree efficiency to an To apply a known Murphree efficiency to an entire column, it is necessary only to replace entire column, it is necessary only to replace the true equilibrium curve ythe true equilibrium curve yee versus x versus xee by an by an

effective equilibrium curve y’effective equilibrium curve y’ee versus x versus xee, who, who

se ordinates are calculated from the equation se ordinates are calculated from the equation (21.70).(21.70).

efficiencyplateMurphree

yyyy

yyyy

yy

yy

ac

ab

M

eMe

nnMnn

nn

nnM

)70.21()(

)70.21()( 11

1

1

5

•How to plot the effective equilibrium curve?How to plot the effective equilibrium curve?

•Giving xGiving xnn, find y*, find y*nn from equilibrium curve and y from equilibrium curve and yn+1n+1 fr fr

om operating line, calculate yom operating line, calculate ynn from equation (21.70), from equation (21.70),

then plot point (xthen plot point (xnn, y, ynn) …) …

•How to determine the number of actual plates?How to determine the number of actual plates?

•The reboiler is not subject to a discount for plate efficThe reboiler is not subject to a discount for plate efficiencyiency, and the true equilibrium curve is used for the la, and the true equilibrium curve is used for the last step in the stripping section.st step in the stripping section.

6

How many How many actual plates actual plates are needed in are needed in the left the left construction? construction?

7

How many How many actual plates actual plates are needed in are needed in the left the left construction? construction?

No fractional actual plate, thenNo fractional actual plate, then

8

•Murphree efficiency can be different from indiMurphree efficiency can be different from individual platesvidual plates, and this is more accurate to calcu, and this is more accurate to calculate the number of actual plates than by late the number of actual plates than by overall overall efficiency of a columnefficiency of a column 全塔效率全塔效率 ..

9

3. BATCH DISTILLATION3. BATCH DISTILLATION （（间歇精馏间歇精馏 ）） [pp.700-70[pp.700-703]3]1)Simple Distillation1)Simple Distillation 简单蒸馏简单蒸馏 (( 自学自学 ))

• (1)Flow diagram (Refer to Fig.21.37)(1)Flow diagram (Refer to Fig.21.37)

)(),( 00 Fnxx F

Batch FeedBatch Feed间歇进料间歇进料

CondenserCondenserCooling waterCooling water

Product Product receiverreceiver

Bxx 1StillStill

10

•There are many cases where There are many cases where batch distillationbatch distillation 间歇精间歇精馏馏 is preferred, particularly in the is preferred, particularly in the food and pharmacfood and pharmaceutical eutical industries. industries. Batch distillationBatch distillation is used when is used when small small amounts of productamounts of product are made in a pilot plant to provi are made in a pilot plant to provide samples for product sampling or testing.de samples for product sampling or testing.

•Features: 1)Unsteady state; 2)Without reflux; 3)One-Features: 1)Unsteady state; 2)Without reflux; 3)One-stage partial vaporization process. stage partial vaporization process. The vapor leaving The vapor leaving the still at any time is in equilibrium with the liquid in the still at any time is in equilibrium with the liquid in the still. the still. [[yy is in equilibrium with is in equilibrium with xx.].]

11

)(),( 00 Fnxx F

Batch FeedBatch Feed

CondenserCondenserCooling waterCooling water

Product Product receiverreceiver

Bxx 1

12

(2)Rayleigh equation(2)Rayleigh equation （瑞利方（瑞利方程）程）

)83.21(ln

)82.21(

)(

)80.21(

,

0

11

0

1

0 n

n

xy

dx

n

dn

xy

dx

n

dn

ydnxdnndx

ydnxnddn

nxn

timegivenaAt

x

x

n

n

A

A

HeaterHeater

Eq.(21.83) is known as the Rayleigh equation. Eq.(21.83) is known as the Rayleigh equation. dx/(y-x)dx/(y-x) can be integrated graphically or numerically using eqcan be integrated graphically or numerically using equilibrium data (uilibrium data (y~xy~x equilibrium relationship). equilibrium relationship).

13

(3) For an ideal mixture,(3) For an ideal mixture,

That is,That is,

BecauseBecause

oror

14

After integration between limitsAfter integration between limits

15

Number of moles of product: DNumber of moles of product: D

EXAMPLE 21.9.EXAMPLE 21.9.

DMxy Average concentration of product:Average concentration of product:

16

Advantages.Advantages. The advantages of batch distilla The advantages of batch distillation are that several products can be made frtion are that several products can be made from a single unit, and can effectively handle om a single unit, and can effectively handle slsludges udges and solids.and solids.

Disadvantages.Disadvantages. For a given product rate, the For a given product rate, the equipment is larger. It requires more operatoequipment is larger. It requires more operator attention, uses more energy, and because it r attention, uses more energy, and because it is a is a dynamic processdynamic process, is harder to control and , is harder to control and model.model.

17

2)Batch Distillation with reflux2)Batch Distillation with reflux 间歇精馏间歇精馏 // 有回有回流的间歇蒸馏流的间歇蒸馏 [pp.702-703][pp.702-703]

• In many cases, a rectifying column with refluIn many cases, a rectifying column with reflux is used to improve the performance of the bx is used to improve the performance of the batch still. atch still.

• Flow diagram.Flow diagram.

• Features:Features: Unsteady state; Unsteady state; Only rectifying section in Only rectifying section in the column.the column. Product concentration differs from diff Product concentration differs from different operations.erent operations.

18

• Two basic operation methods:Two basic operation methods:

• (a). (a). Keep the top composition (xKeep the top composition (xDD) constant) constant b b

y y increasing the reflux ratioincreasing the reflux ratio as the compositi as the composition of the liquid in the reboiler changes. on of the liquid in the reboiler changes.

• (b). (b). Fix the reflux ratioFix the reflux ratio and and let the overhead let the overhead product purity vary with timeproduct purity vary with time, stopping the d, stopping the distillation when the amount of product or the istillation when the amount of product or the average concentration in the total product reaverage concentration in the total product reaches a certainaches a certain(( 某个某个 )) value. value.

19

• Calculation and analysis for the operation of Calculation and analysis for the operation of a distillation column.a distillation column.

• The plate column has been establishedThe plate column has been established, and t, and the following data are given: he following data are given: Equilibrium relaEquilibrium relationship; xtionship; xFF; q; N and ; q; N and MM (N (Nactualactual)). .

• In order to obtain overhead and bottom prodIn order to obtain overhead and bottom products with required mole fractions of xucts with required mole fractions of xDD and x and x

BB, , the reflux ratio Rthe reflux ratio RDD and the feed plate positi and the feed plate positi

on must be determinedon must be determined..• Calculation method: Calculation method: Trial and errorTrial and error （反复试验）（反复试验） ..

20

• Attentions of Operation analysis of distillation Attentions of Operation analysis of distillation column:column:

(a) Suitable feed plate position;(a) Suitable feed plate position;

(b) Keep material balance: If D and B changed, x(b) Keep material balance: If D and B changed, xDD and and

xxBB will be changed too. will be changed too.

(c) (c) Reflux ratio is the key measure of controlling produReflux ratio is the key measure of controlling product quality.ct quality. [For example, if N is given, R [For example, if N is given, RDDxxDD, x, xBB

.].]

(d) If x(d) If xFF changed, feed plate position should be change changed, feed plate position should be change

d too.d too.

21

4. AZEOTROPIC AND EXTRACTIVE DISTI4. AZEOTROPIC AND EXTRACTIVE DISTILLATION [pp.131]LLATION [pp.131] （（恒沸精馏和萃取精馏恒沸精馏和萃取精馏））

• When When relative volatilityrelative volatility （相对挥发度）（相对挥发度） near or equal to unitnear or equal to unityy, the separation of components is difficult by conve, the separation of components is difficult by conventional distillation, or impossible because of ntional distillation, or impossible because of azeotrazeotropeope 恒沸混合物恒沸混合物 formation. formation.

• Basic principles of azeotropic and extractive distillaBasic principles of azeotropic and extractive distillationtion: Adding a third component(: Adding a third component(solventsolvent) to ) to increase increase the relative volatilitythe relative volatility of the original components, so of the original components, so that the mixtures with relative volatility near or equthat the mixtures with relative volatility near or equal to unity can be separated by conventional distillaal to unity can be separated by conventional distillation.tion.

22

(1)(1) Azeotropic distillationAzeotropic distillation 恒沸精馏恒沸精馏• Materials called Materials called entrainersentrainers(( 夹带剂夹带剂 ))(third compone(third compone

nt) are added to the original mixture, and usually a nt) are added to the original mixture, and usually a new new azeotrope with lowest-boiling pointsazeotrope with lowest-boiling points 有最低恒有最低恒沸点的恒沸物沸点的恒沸物 is formed. The new azeotrope will of is formed. The new azeotrope will of course contain one or more of the feed components, course contain one or more of the feed components, and leaving behind component(s) which may be recand leaving behind component(s) which may be recovered in the pure state.overed in the pure state.

• Example: Recovery and purification of ethanol and Example: Recovery and purification of ethanol and water. Flow diagram…water. Flow diagram…

• Selection of Selection of entrainers.entrainers.

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(2)(2) Extractive distillationExtractive distillation 萃取精馏萃取精馏• A solvent is added to the distillation tower to increaA solvent is added to the distillation tower to increa

se relative volatility of key components in the feed se relative volatility of key components in the feed mixture. The effective mixture. The effective extractive solventextractive solvent 萃取剂萃取剂 will will selectively interact with one (or more) of the composelectively interact with one (or more) of the components, thereby increasing relative volatilities.nents, thereby increasing relative volatilities.

• Example:Use of Example:Use of furfuralfurfural 糠醛糠醛 to permit the separatto permit the separation of ion of butadienebutadiene 丁二烯丁二烯 from a mixture containing from a mixture containing butanebutane 丁烷丁烷 and and butenesbutenes 丁烯丁烯 . Flow diagram…. Flow diagram…

• Selection of Selection of extractiveextractive solventssolvents..