ERT 318 UNIT OPERATIONS

Transcription

1 ERT 318 UNIT OPERATIONS DISTILLATION W. L. McCabe, J. C. Smith, P. Harriot, Uit Operatios of Chemical Egieerig, 7 th editio,

2 Outlie: Batch distillatio (pg. 724) Cotiuous distillatio with reflux (pg. 666) Material balace i plate colum (pg. 670) McCabe-Thiele Method (pg. 672) Determiatio of the umber of plates usig ethalpycompositio diagram (pg. 694) Itroductio to multicompoet distillatio Phase Equilibria i Multicompoet Distillatio (Pg. 737) Flash Distillatio of Multicompoet Mixtures (pg. 741) Fractioatio of Multicompoet Mixtures (pg. 742) Azeotropic ad Extractive Distillatio (pg. 759) 2

3 Batch Distillatio Vapor Fig : Simple distillatio i a batch still Ofte used i small plats to recover volatile products from liquid. Vapor: - equilibrium with the liquid i the still - Richer with volatile compoet - compositio liquid & vapor are ot costat 3

4 Batch Distillatio 4

5 Batch Distillatio 5

6 Batch Distillatio 6

7 Batch Distillatio 7

8 Cotiuous Distillatio with Reflux Ofte used for large-scale productio, far more commo tha batch distillatio. y = vapor leavig plate x = Liquid leavig plate y +1 = vapor eterig plate x +1 = liquid eterig plate V = vapor phase y = cocetratio of vapor L = Liquid phase x = cocetratio of liquid T -1 < T < T +1 8 Fig. 21.3: Material-balace diagram for plate Fig. 21.4: Boilig-poit diagram showig rectificatio o ideal plates

9 Cotiuous Distillatio with Reflux Combiatio rectificatio ad strippig Why feed is i the cetral? What is feed plate? To purify by repeated distillatio Reflux provides the dowflowig liquid i the rectifyig sectio that is eeded to act o the upflowig vapor. No reflux o rectificatio Physical separatio where more compoets are removed from a liquid stream by vapor stream If o azeotropes,both overhead ad bottom products obtaied i ay desired purity if eough plates ad adequate reflux are provided. Fig. 21.5: Cotiuous fractioatig colum with rectifyig ad strippig sectios. 9

10 Cotiuous Distillatio with Reflux Material Balaces i Plate colums Overall material balace for two-compoet systems Total material balace: F = D + B Compoet A balace: Fx F = Dx D + Bx B Elimiatig B: D/F = (x F -x B ) / (x D -x B ) Elimiatig D: B/F = (x D -x F ) / (x D -x B ) (21.6) (21.7) (21.8) (21.9) Eq & 21.9 are true for all values of the flows of vapor ad liquid withi the colum Net flow rates: D = V a -L a D = V +1 -L Dx D = V a y a L a x a = V +1 y +1 -L x B = L b V b = L m Vm+1 Bx B = L b x b V b y b = L m x m - V m+1 y m+1 (21.10) (21.11) (21.12) (21.13) (21.14) Fig. 21.6: Material-balace diagram for cotiuous fractioatio colum

11 Cotiuous Distillatio with Reflux a a a a V x L y V x V L y D V Dx x V L y D L Dx x D L L y D 1 Operatig lies : Because there are 2 sectios i the colum, there are also 2 operatig lies; 1 rectifyig sectio ad 1 strippig sectio. Compoet A balace: Fx F = Dx D + Bx B From Eq. (21.7): From Eq. (21.9): (21.15) (21.16) (21.17) The slope defied by Eq. (21.6) the ratio of liquid to the vapor stream: by elimiatig V +1

12 Cotiuous Distillatio with Reflux Operatig lies: Material balace over cotrol surface II, below the feed plate: V y m1 m1 L m x m Bx B (21.18) I a differet form, this becomes y m1 L V m m1 x m Bx V B m1 (21.19) This is the equatio for the operatig lie i the strippig sectio. Slope =liquid flow / vapor flow. Elimiatig V m+1 from Eq (21.19) & (21.13) gives: y m1 L Lm m B x m BxB L B m (21.20)

13 Cotiuous Distillatio with Reflux Number of ideal Plates; McCabe-Thiele Method ASPEN : Computer desig program used to idetify the umber of plates required for a distillatio problem. Mc Cabe-Thiele Method : A simplified graphical method for calculatig the umber of plates. Whe the operatig lies used Eqs. (21.17) ad (21.20) are plotted withi the equilirium curve o the xy diagram, the McCabe-Thiele step-by-step costructio ca be used to compute the umber of ideal plates Eqs. (21.17) ad (21.20), show that uless L ad L m are costat, the operatig lies are curved ad ca be plotted oly if the chage i these iteral streams with cocetratio is kow. Ethalpy balaces are required i the geeral case to determie the positio of a curved operatig lie. See Example Tutorial 1: Problem 21.11

14 Example 21.2

15 Example 21.2: Solutio

16 Example 21.2: Solutio

17 Example 21.2: Solutio

18 Example 21.2: Solutio

19 Example 21.2: Solutio

20 Example 21.2: Solutio

21 Determiatio of the umber of plates usig ethalpycompositio diagram

22 Ethalpy Balaces Variatios i V ad L streams deped o the ethalpies. Ethalpy data may be available from a ethalpy-cocetratio diagram (e.g., Fig. 21.2) or data bak from i computer program. T b, bezee = 80 o C T b, toluee = o C Temp. rage = o C Ofte distillatio colums are desiged usig computers, the basic ethalpy balace equatio are give. Refer to Example 21.5, which illustrate the small differece this makes i the McCabe-Thiele diagram for a typical ideal system. Bezee-toluee solutios are ideal. Fig : Ethalpy-cocetratio diagram for bezee-toluee at 1atm. For: Liquid bubble poit Vapor dew poit Temperature for x = 0.5 ad y = 0.5 is ot the same! (Refer to ext slide) Slight curvature i Fig is due to the oliear chage i the bubble-poit ad dewpoit with mole fractio bezee. 22

23 Ethalpy Balaces Figure 1: T-x-y diagram for Bezee-Toluee 1atm 23

24 Ethalpy Balaces Cosider a overall ethalpy balace for system show i Fig (REPEAT) FH F + q r = DH D + BH B +q c H F = ethalpy of feed H D = ethalpy of overhead product H B = ethalpy of bottom product q c = heat removed from codeser = q r (21.48) Fig. 21.6: Material-balace diagram for cotiuous fractioatio colum For give feed ad product streams, oly 1 of heat effects, q r or q c. Normally, q c is chose i desigig a colum to correspod to the desired reflux ratio ad moles of the overhead vapor. The, q r ca be calculated usig Eq. (21.48). I operatig colum, q r is ofte varied to chage the vapor flow rate ad reflux ratio, ad chages i q c the follow. 24

25 Ethalpy Balaces Ethalpy balaces i rectifyig ad strippig sectios 25

26 Ethalpy Balaces Ethalpy balaces i rectifyig ad strippig sectios 26

27 Ethalpy Balaces Ethalpy balaces i rectifyig ad strippig sectios 27

28 Example 21.5 A mixture of 50 mol percet bezee ad toluee is to be separated by distillatio at atmospheric pressure ito products of 98% purity usig a reflux ratio 1.2 times the miimum value. The feed is liquid at the boilig poit. Use ethalpy balaces (Table 21.3) to calculate the flows of liquid ad vapor at the top, middle, ad bottom of the colum, ad compare these values with those based o molal overflow. Estimate the differece i the umber of theoretical plates for the methods. Table 21.3: Data for Example 21.5 Compoet Ethalpy of vaporizatio, cal/g mol Specific heat at costat pressure, cal/g mol. C Liquid Vapor Boilig poit, C Bezee Toluee 7,360 7,

29 Tutorial 2 Problems: Due Thursday, 08/10/