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FLASH DISTILLAN

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Page 1: FLASH DISTILLAN

CENG 221

Lecture 2. Simple Distillation Processes (4.5 h)

Learning Objectives:

(1) Simple batch distillation

(2) Binary flash distillation

(3) Sizing of flash distillation drum

Learning Guides:

(1) Lecture handouts

(2) Chapters 11.1-11.2, 3.1-3.6 and 2 of Textbook:

Equilibrium-Staged Separations

(3) Please review the notes on VLE, bubble and dew point

calculations

CENG 221

Lecture 2. Simple Distillation Processes (4.5 h)

Learning Objectives:

(1) Simple batch distillation

(2) Binary flash distillation

(3) Sizing of flash distillation drum

Learning Guides:

(1) Lecture handouts

(2) Chapters 11.1-11.2, 3.1-3.6 and 2 of Textbook:

Equilibrium-Staged Separations

(3) Please review the notes on VLE, bubble and dew point

calculations

Simple Batch Distillation

Used only for small scale processes,especially in pharmaceuticals andspecialty products.

Page 2: FLASH DISTILLAN

Simple Batch Distillation

Water in

Water out

F = W + DF Xf = W Xw + D Xd

Rayleigh Equationd(W Xw) = Yd dW = Xw dW + W dXw

dW/W = dXw /(Yd - Xw)

F = W + DF Xf = W Xw + D Xd

Rayleigh Equationd(W Xw) = Yd dW = Xw dW + W dXw

dW/W = dXw /(Yd - Xw)

Simple Batch Distillation

Yd

Xw

Equilibrium

W - dW

dW

Material Balance:(W-dW) Xw + Yd dW = W (Xw + dXw)dW/W = dXw/(Yd-Xw)…………….Rayleigh Equation

Page 3: FLASH DISTILLAN

X(ethanol) X(water) Y(ethanol) Y(water) T(C)0 1 0 1 100

0.019 0.981 0.17 0.83 95.50.0721 0.9279 0.3891 0.6109 890.0966 0.9034 0.4375 0.5625 86.70.1238 0.8762 0.4704 0.5296 85.30.1661 0.8339 0.5089 0.4911 84.10.2377 0.7623 0.5445 0.4555 82.70.2608 0.7392 0.558 0.442 82.30.3273 0.6727 0.5826 0.4174 81.50.3965 0.6035 0.6122 0.3878 80.70.5079 0.4921 0.6564 0.3436 79.80.5198 0.4802 0.6599 0.3401 79.70.5732 0.4268 0.6841 0.3159 79.30.6763 0.3237 0.7385 0.2615 78.740.7472 0.2528 0.7815 0.2185 78.410.8943 0.1057 0.8943 0.1057 78.15

1 0 1 0 78.3

Simple Batch Distillation

Example 9: An alcohol soln. contains 20 mole % ethanol was distilledin a batch distillation unit. The distillation was stopped when thetemperature of the still reaches 90 C. Please determine the following:(1) What is the bubble point temperature of the starting and end

mixtures?(2) What is the concentration of first bubble of vapor formed from

the solution and the last bubble before the distillation was stopped?(3) Determine the amount of liquid remaining in the still if the

starting solution is 1 Kmole.(4) How much distillate was obtain and its average concentration?(5) What is the expected bubble point temperature of the distillate?

75

80

85

90

95

100

105

0 0.2 0.4 0.6 0.8 1

X,Y (ethanol)

T(C

)

(1) Plot the T-x-y data for ethanol-water solution

Page 4: FLASH DISTILLAN

T(C) X Y Y-X 1/(Y-X)100 0 0

95.5 0.019 0.17 0.151 6.62251789 0.0721 0.3891 0.317 3.154574

86.7 0.0966 0.4375 0.3409 2.93341285.3 0.1238 0.4704 0.3466 2.8851784.1 0.1661 0.5089 0.3428 2.91715382.7 0.2377 0.5445 0.3068 3.25945282.3 0.2608 0.558 0.2972 3.36473881.5 0.3273 0.5826 0.2553 3.9169680.7 0.3965 0.6122 0.2157 4.63606979.8 0.5079 0.6564 0.1485 6.73400779.7 0.5198 0.6599 0.1401 7.13775979.3 0.5732 0.6841 0.1109 9.017133

78.74 0.6763 0.7385 0.0622 16.0771778.41 0.7472 0.7815 0.0343 29.1545278.15 0.8943 0.8943

78.3 1 1

(2) Calculate the parameters in the Rayleigh Equation:

F(x) = 1/(Y-X) vs X

Page 5: FLASH DISTILLAN

(3) Plot F(x) vs X

0

10

20

30

0 0.2 0.4 0.6 0.8

X

F(x

)

(3) Graphical Integration

0

2

4

6

8

10

0 0.1 0.2 0.3

X

F(x

)

Page 6: FLASH DISTILLAN

(3) Graphical Integration

0

2

4

6

8

10

0 0.1 0.2 0.3

X

F(x

)

Numerical IntegrationSimpson Rules:

Area = (Xf - Xi) 1/6 [F(Xi) + 4F(Xm) + F(Xf)]where Xf is the final point

Xi is the initial pointXm is the midpoint given by 0.5Xf + 0.5Xi

Page 7: FLASH DISTILLAN

Simple Batch Distillation

Water in

Water out

Example 10: A 100 g solution of isoamyl alcohol (65 wt.%) and ethanol (35 wt.%) was to be separated using a laboratory still (see figure). Thebinary solution has a relative volatility of 2.25. At the end of thedistillation, 30 g of solution was left in the still.(1) What is the concentration of the first drop of distillate collected

from the distillation unit?(2) What is the concentration of isoamyl alcohol in the remaining

solution in the still?(3) What is the concentration of ethanol in the last drop of distillate

collected?(4) How much distillate is produced and what is its ethanol content?(5) If an isoamyl concentration of 95 % is needed, what is maximum

yield one can obtain?

(1) Plot y vs x

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mass fraction of EtOH)

y (m

ass

frac

tion

of E

tOH

)

Isoamyl alcohol-ethanol solution

Page 8: FLASH DISTILLAN

x y y-x 1/(y-x)0 0

0.04 0.085714 0.045714 21.8750.08 0.163636 0.083636 11.956520.12 0.234783 0.114783 8.7121210.16 0.3 0.14 7.142857

0.2 0.36 0.16 6.250.24 0.415385 0.175385 5.7017540.28 0.466667 0.186667 5.3571430.32 0.514286 0.194286 5.1470590.36 0.558621 0.198621 5.034722

0.4 0.6 0.2 50.44 0.63871 0.19871 5.0324680.48 0.675 0.195 5.1282050.52 0.709091 0.189091 5.2884620.56 0.741176 0.181176 5.519481

(2) Calculate and plot 1/y-x vs x (Rayleigh equation)

0

5

10

15

20

25

0 0.2 0.4 0.6

x (mass fraction of EtOH)

F(x

) =

1/y

-x

0

5

10

15

20

25

0 0.2 0.4 0.6

x (mass fraction of EtOH)

F(x

) =

1/y

-x

Isoamyl alcohol-ethanol solution

Page 9: FLASH DISTILLAN

Ethanol + Water

LIQUID

VAPOR

Ethanol

Water

Flash DistillationSingle stage distillation process for continuous separation.

Continuous FEEDContinuous

DISTILLATE

Flash DistillationUsed mainly for simple separation. Separate component with largedifference in volatility.Example: distilled water

Pb Pc PdPa <<> >

pump

heater

throttlevalve

Tb Tc TdTa ≥~ <

Liquid productL, x, hL

Vapor productV, y, Hv

TdrumPdrum

Q

TF, PF, hF

FeedF, z,T1, P1

Page 10: FLASH DISTILLAN

Degree of Freedom (F)

F = {total # of independent variables} - {total # of equations}

Variables No Relation NoFEEDF, feed rate 1 Enthalpy (H) 3zi, feed composition C-1 VLE data (y = Kx) CT1, temperature 1 Mass Balance CP1, pressure 1 Energy Balance 1

LIQUIDL, liquid flow rate 1xi, liquid composition C-1hL, liquid enthalpy 1

VAPORV, liquid flow rate 1yi, liquid composition C-1HL, liquid enthalpy 1

STILLTdrum, temperature 1Pdrum, pressure 1Q, heating rate 1

F = {total # of independent variables} - {total # of equations}

Variables No Relation NoFEEDF, feed rate 1 Enthalpy (H) 3zi, feed composition C-1 VLE data (y = Kx) CT1, temperature 1 Mass Balance CP1, pressure 1 Energy Balance 1

LIQUIDL, liquid flow rate 1xi, liquid composition C-1hL, liquid enthalpy 1

VAPORV, liquid flow rate 1yi, liquid composition C-1HL, liquid enthalpy 1

STILLTdrum, temperature 1Pdrum, pressure 1Q, heating rate 1

Variables = 3C + 8Equations = 2C + 4degree of freedomF = C + 4

Variables = 3C + 8Equations = 2C + 4degree of freedomF = C + 4

Degree of Freedom (F)

Flash Distillation: F = C + 4

Binary flash distillation# components: C = 2

F: 2 + 4 = 6 = number of parameters that needed to bespecified in order to solve a flash distillation problem.

= number of design parameters in order to build flash distillation unit.

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:

Binary flash distillation# components: C = 2

F: 2 + 4 = 6 = number of parameters that needed to bespecified in order to solve a flash distillation problem.

= number of design parameters in order to build flash distillation unit.

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:

Page 11: FLASH DISTILLAN

Flash Distillation (Sequential Solution -1)

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:condition of the drum-1: Tdrum, y or x

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:condition of the drum-1: Tdrum, y or x

Example 11: 1000 mole/h of 0.4 mole fraction ethanol-water solution(T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa.If the concentration of ethanol in the vapor leaving the flash drum is0.6 mole fraction, what is the Tdrum, x, L, V and Qheater?

Page 12: FLASH DISTILLAN

Flash Distillation (Sequential Solution -1)

75

80

85

90

95

100

105

0 0.2 0.4 0.6 0.8 1

X,Y (ethanol)

T(C

)

Page 13: FLASH DISTILLAN

Flash Distillation (Sequential Solution -2)

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:condition of the drum-2: V, L, f = V/F, q = L/F or L/V

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:condition of the drum-2: V, L, f = V/F, q = L/F or L/V

Example 10: 1000 mole/h of 0.4 mole fraction ethanol-water solution(T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa.Determine the flowrate and composition of the products leaving the flash drum and the Tdrum given:(a) amount of vapor produced is 500 mole/h,(b) the fraction of liquid remaining is 0.7,(c) for (b) also calculate the heat needed to achieve the distillation.

(1) Plot VLE data in form of McCabe-Thiele diagram:

Operating Equation:OMB: F = V + LCMB: Fz = Vy + Lx solve the 2 eqns and set in form of y = f(x)

y = -(L/V)x + (F/V)z = mx + b (equation for a line)y = -(1-f/f)x + z/fy = -(q/1-q)x + z/(1-q)

slope, m = -L/V = -(1-f/f) = -(q/1-q)y-intercept, b = (F/V)z = z/f = z/(1-q)

Operating Equation:OMB: F = V + LCMB: Fz = Vy + Lx solve the 2 eqns and set in form of y = f(x)

y = -(L/V)x + (F/V)z = mx + b (equation for a line)y = -(1-f/f)x + z/fy = -(q/1-q)x + z/(1-q)

slope, m = -L/V = -(1-f/f) = -(q/1-q)y-intercept, b = (F/V)z = z/f = z/(1-q)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

X (ethanol)

Y (

eth

an

ol)

Flash Distillation (Sequential Solution -2)

Page 14: FLASH DISTILLAN

Flash Distillation (Sequential Solution -2)

75

80

85

90

95

100

105

0 0.2 0.4 0.6 0.8 1

X,Y (ethanol)

T(C

)

Page 15: FLASH DISTILLAN

Flash Distillation (Simultaneous Solution)

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:condition of the feed entering the drum: TF or hF

Parameters usually specified:(1) Feed conditions:

F (feed rate), z (feed, composition)T1, P1

(2) Still condition: Pdrum

The last parameters will dictate the solution procedure:condition of the feed entering the drum: TF or hF

Example 11: 1000 mole/h of 0.4 mole fraction ethanol-water solution(T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa.Determine the flowrate, composition and enthalpy of the productsleaving the flash drum and the Tdrum given:(a) TF is 83 C(b) enthalpy of the feed entering the drum, hF is 150 kcal/kg

Different Representation of VLE Data

(6) Ponchon-Savarit Diagram:

H-x-y diagram

Page 16: FLASH DISTILLAN

Sizing of Flash Drum

D

D = (4Ac/π)0.5

Ac (ft2) = W(lbmol/h)MWvapor(lbm/lbmol)uperm(ft/s)(3600)ρv(lbm/ft3)

uperm = Kdrum(ρL - ρv/ρv)0.5

Kdrum = exp(A + BlnFlv + C(lnFlv)2

+ D (lnFlv)3 + E (lnFlv)4)where: A = -1.87748

B = -0.81458C = -0.18707D = -0.01452E = -0.00101Flv = WL/Wv (ρv/ρL)0.5

Page 17: FLASH DISTILLAN

Sizing of Flash Drum

D

hv = 0.5D + 36” > 48”

hf = 0.5D + 12”

hL = 4Vs/πD2

Note: L = hv + hf + hL3 < L/D < 5

Example 12: Determine the size (i.e., diameter and height) of theflash drum needed to separate 1000 mole/h of 0.4 mole fraction ethanol-water solution (T = 24 C, P = 101.3 kPa) enters a flash drum operated at 101.3 kPa. If the the fraction of liquid remaining is 0.7.

Sizing of Flash Drum

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

X (ethanol)

Y (

eth

an

ol)

Page 18: FLASH DISTILLAN

Sizing of Flash Drum

75

80

85

90

95

100

105

0 0.2 0.4 0.6 0.8 1

X,Y (ethanol)

T(C

)

Flash Drum in Series (Cascade)

1 2 3

V1V2 =(F3)

V3

L1 (=F2)L2 L3

F1

Example 13: Determine the composition and flowrate of each product streams leaving the flash drums 1, 2 and 3. A 0.3 mass fractionethanol-water solution was feed to the first drum in the cascade.The temperature of the first drum was 89°C. The feed to the seconddrum was preheated so that its enthalpy (hf2) is 200kcal/kg. The vaporfraction in the last drum is 0.2.

Please use the Ponchon-Savarit Diagram for your solution.

Page 19: FLASH DISTILLAN

Flash Drum in Series (Cascade)

1

V1

L1 (=F2)

F1

Page 20: FLASH DISTILLAN

Flash Drum in Series (Cascade)

2

V2 =(F3)

L1 (=F2)L2

Page 21: FLASH DISTILLAN

Flash Drum in Series (Cascade)

3

V2 =(F3)V3

L3