ceng 221 lecture 4. multi-component distillation (4.5 h) learning objectives: (1) multi-component...

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CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column distillation Fenske Equation Underwood Equation Gilliland Correlation (3) Column sizing Column diameter Column height (4) Introduction to packed column Learning Guides: (1) Lecture handouts (2) Chapters 3.3, 7.1, 9.1-9.3, 12.1, 12.3, 13.3 of Textbook: Equilibrium-Staged Separations

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Page 1: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

CENG 221

Lecture 4. Multi-component Distillation (4.5 h)

Learning Objectives:

(1) Multi-component flash distillation calculation

(2) Multi-component column distillationFenske EquationUnderwood EquationGilliland Correlation

(3) Column sizingColumn diameterColumn height

(4) Introduction to packed column

Learning Guides:

(1) Lecture handouts

(2) Chapters 3.3, 7.1, 9.1-9.3, 12.1, 12.3, 13.3 of Textbook:

Equilibrium-Staged Separations

CENG 221

Lecture 4. Multi-component Distillation (4.5 h)

Learning Objectives:

(1) Multi-component flash distillation calculation

(2) Multi-component column distillationFenske EquationUnderwood EquationGilliland Correlation

(3) Column sizingColumn diameterColumn height

(4) Introduction to packed column

Learning Guides:

(1) Lecture handouts

(2) Chapters 3.3, 7.1, 9.1-9.3, 12.1, 12.3, 13.3 of Textbook:

Equilibrium-Staged Separations

Page 2: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Multi-component Column Distillation

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

A 2000 kmole/h feed containing benzene (0.2), toluene (0.3) and xylene (0.5) are to be separated using the distillation column shown above. 90 percent of the toluene is to be recovered at the distillate and 98 percent of xylenes is produced at the bottom.(a) label the key and non-key components(b) what is the value of D and B?(b) what is the distillate and bottom composition?

Page 3: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Column Distillation

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

A 100 kmole/h feed containing methanol (0.3) and water (0.7) is to be separated using the distillation column shown above. 90 percent of the methanol is to be recovered at the distillate and 90 percent of water is produced at the bottom.(a) label the key and non-key components(b) what is the value of D and B?(c) what is the distillate and bottom composition?

Page 4: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Fenske Equation (Total Reflux)

Calculates for Nmin

Nmin = Ln [(xA/xB)d/(xA/xB)b]

Ln AB

where xA is concentration of key component A (usually LKC) xB is concentration of key component B (usually HKC

For binary components

Nmin = Ln [(xA/1-xA)d/(xA/1-xA)b]

Ln AB

Page 5: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Underwood Equation (Minimum Reflux)

Calculates for Rmin

Vfeed = F(1-q) =i -

iFzi

Calculates for Vmin

Vmin =i -

iDxi

Page 6: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Column Distillation

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

Given that the methanol/water = 3.5, determine the minimum number of equilibrium stages (Nmin) needed for the separation.

- Use McCabe-Thiele method- Use Fenske Equation

Page 7: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mole frac. methanol)

y (m

ole

fra

c. m

eth

ano

l)

McCabe-Thiele Method

=3.5

=3.5

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mole frac. methanol)

y (m

ole

fra

c. m

eth

ano

l)

Page 8: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Column Distillation

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

Given that feed is saturated liquid, determine the minimum reflux (Rmin) - Use McCabe-Thiele method

- Use Underwood Equation

Page 9: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mole frac. methanol)

y (m

ole

fra

c. m

eth

ano

l)

McCabe-Thiele Method

=3.5

=3.5

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mole frac. methanol)

y (m

ole

fra

c. m

eth

ano

l)

Page 10: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Column Distillation

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

Given that the reflux is 2 Rmin find the number of trays needed for the separation (N) - Use McCabe-Thiele method

- Use Gillaland Correlation

Page 11: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mole frac. methanol)

y (m

ole

fra

c. m

eth

ano

l)

McCabe-Thiele Method

=3.5

=3.5

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

x (mole frac. methanol)

y (m

ole

fra

c. m

eth

ano

l)

Page 12: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Example 25: The distillation column shown in the figure below was used for the separation of 0.5 mole fraction methanol-water solution. The desired distillate and bottom products are 0.20 and 0.9, respectively. The feed enters the column as a subcooled liquid that condenses 2 moles of vapor per mole of feed.

QC

QR

F, z, hf

D, xD= 0.9, hD

B, xB= 0.2, hB

Q=0

Reflux ratio = L0/D

Boilup ratio = Vn+1/B

(n)

Reboiler

Condenser

10 Kmole/min,0.5Subcooled liquid

= 2 Rmin

(a) What is q-value of the feed? Plot the feed line.(b) What is the number of equilibrium stages?(c) What is the actual number of stages if the EMV = 0.5?(d) Solve the problem using Fenske, Gilliland and Underwood methods.

Page 13: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Multi-component Column Distillation

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

Example 26. A mixture containing 0.2 mole frac. Benzene (BT = 2.25), 0.3 toluene (TT = 1.0), 0.1 xylene (XT = 0.33) and 0.4 cumene (CT = 0.21) is to be separated by a distillation column equipped with partial reboiler and total condenser. The 2-phase feed contains 30 % vapor. 99.8 % of the cumene is to be recovered at the bottom and 99.5% of the toluene is to be recovered at the distillate. If the reflux ratio is fixed at 3Rmin determine the number of stages needed for the separation.

Page 14: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Sizing of Distillation Column

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

Number of separation trays (N)Height of column (hC = N*tray spacing)Column diameter (dc)

Page 15: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Sizing of Distillation Column Diameter

uflood = K [(L - V)/V]0.5 ft/s

(1) decide on the tray spacing values using increases by increment of 6 inches from 6”-36”

(2) use the graphical correlation that relates the constant Csb for different tray spacing to Flv

Flv = WL/Wv [L/V]0.5

note: W is mass flow rates

(3) calculate the flooding velocity. This is the maximum flowrates allowable that will prevent excessive entrainment of liquid.

(4) actual operation velocity is usually lower than flooding velocity where 0.65 < (fraction) < 0.9

uop = (fraction) uflood ft/s

(5) finally, uop is related to the column diameter by:

uop = V(MWav)/3600VAnet ft/s

Page 16: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Sizing of Distillation Column

QC

QR

F, hf

D, xDi

B, xBi

Q = 0

(n)

Partial Reboiler

Total Condenser

zi

R

Example 27a. Ethanol-water solution was to be separated using a tray distillation column equipped with a total condenser and a partial reboiler. 50 Kmole/h of saturated liquid feed (0.4 mole frac. ethanol) was to be separated to obtain a distillate containing 0.6 mole frac. Ethanol and a bottom of 0.1 mole frac. The reflux ratio is 1.8 Rmin. The distillation company suggested a tray spacing of 24” for optimum operation. Please calculate the diameter and height of the distillation column if sieve trays were used as separation stage.

Page 17: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

X (ethanol)

Y (

eth

ano

l)

VLE Data

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

X (ethanol)

Y (

eth

ano

l)

Page 18: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Multi-component Flash Distillation

Pb Pc PdPa >>> >

pump

heater

throttlevalve

Tb Tc TdTa ~ <

Liquid productL, xi, hL

Vapor productV, yi, Hv

Tdrum

Pdrum

Q

TF, PF, hF

FeedF, zi,T1, P1

Rachford-Rice Equation

f(V/F) =(Ki-1)zi

1 + (Ki-1)(V/F)i=1

c

= 0

Page 19: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Solution Methods for Rachford-Rice Equation

f(V/F) =(Ki-1)zi

1 + (Ki-1)(V/F)i=1

c

= 0

(1) Secant method (a) choose (V/F)1 such that f(V/F)1 < 0 (V/F)2 such that f(V/F)2 > 0 (b) use linear interpolation to determine V/F at f(V/F) = 0 (c) repeat (a) using new V/F value from (b) until f(V/F) =0

(2) Newtonian convergence read page 55 of Textbook for details

Page 20: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Multi-component Flash Distillation

F = 1000 kmole/h

zc3 = 0.3

znC4 = 0.1

znc5 = 0.15

znc6 = 0.45

V, yi

L, xi

T = 50°CP = 200 kPa

Example 26: Determine the amount and composition of the productsleaving the flash distillation shown below.

Page 21: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

VLE Data

Page 22: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

Multi-component Flash Distillation

F = 1000 kmole/h

zc3 = 0.25

znC4 = 0.15

znc5 = 0.15

znc6 = 0.45

V, yc3 = 0.7

L, xi

T = ?P = 200 kPa

Example 27: Determine the operating temperature for the flash drum if the composition of propane in the vapor is 0.7.

Page 23: CENG 221 Lecture 4. Multi-component Distillation (4.5 h) Learning Objectives: (1) Multi-component flash distillation calculation (2) Multi-component column

VLE Data