Distillation Column Design, Assignments of Industrial design

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CH 4050 UNIT OPERATION II

Distillation Column Design Assignment

J.S. Ahamed | 170019G

Department of Chemical and Process Engineering

Table of Contents

1. Problem

Design a distillation column with a total condenser and a partial reboiler for the following

separation.

Data:

System : Benzene-Ethylbenzene

Operating pressure : 1 bar

Feed rate : 80 kmol/hr

Feed condition : saturated vapour

Feed composition: 60 mol% benzene

Distillate composition: 95 mol% benzene

Bottom product composition : 4 mol% benzene

Column type : Sieve tray column

Operating condition : 70 Percent of flooding

Obtain equilibrium data for the system from ‘Vapour Pressure Data’ available in Moodle page.

Estimate the tray and/or column efficiency and assume a suitable reflux ratio, down comer type

and area and calculate the Actual number of stages required , the Height and the Diameter of

the column, Size and the Number of holes in a tray, tray spacing and the feed tray location.

Check the design for satisfactory operation.

2. Constructing the equilibrium curve

• 1. Problem
• 1. Constructing the equilibrium curve
• 1. Finding the distillate and bottom product
• 1. Determination of minimum reflux ratio
  • 4.1. q line equation
  • 4.2. Top operating line equation (at minimum reflux ratio condition)
• 1. Constructing the top operating line (TOL) for operating reflux ratio
• 1. Constructing the bottom operating line (BOL) for operating reflux ratio
• 1. Determination of ideal number of stages
• 1. Calculation of flowrates....................................................................................................
  • 8.1. Material balance for rectifying section......................................................................
  • 8.2. Material balance for stripping section
• 1. Liquid and Vapour Density calculation
  • 9.1. Rectifying section densities
  • 9.2. Stripping section densities.........................................................................................
• 1. Calculation of column diameter
  • 10.1. Diameter of rectifying section
  • 10.2. Checking the assume tray spacing value
  • 10.3. Calculation of diameter of stripping section..........................................................
• 1. Calculation of hole diameter and area
• 1. Number of Holes in a tray at rectifying section
• 1. Number of Holes in a tray at stripping section
• 1. Flow pattern identification and down comer selection
  • 14.1. Flow pattern of rectifying section
  • 14.2. Flow pattern of stripping section
• 1. Column efficiency calculation
  • 15.1. Fractional area (FA) calculation
  • 15.2. Surface tension number calculation,
  • 15.3. Liquid Schmidt number calculation,
  • 15.4. Reynolds number calculation;
• 1. Calculation of actual number of stages required...........................................................
• 1. Calculation of feed tray location
• 1. Calculation of height of the column (Hc)
• 1. Summary
• 1. References
  • 135.93 4.22 1. Temp °C Vapour Pressure of A Vapour Pressure of B
• 135.936 4.22229381 1.
• 135.818 4.211786408 1. - 135.7 4.2013398 1.
• 135.583 4.190903325 1.
• 135.349 4.170182363 0.
• 135.116 4.149613388 0.
• 134.884 4.1291964 0.
• 133.739 4.029477401 0.
• 132.617 3.93350405 0.
• 131.518 3.841104094 0.
• 129.386 3.666377575 0.
• 127.339 3.504109809 0.
• 125.372 3.353229285 0.
• 120.783 3.019545795 0.
• 116.615 2.738054813 0.
• 112.815 2.498609652 0.
• 109.339 2.293278593 0.
• 106.146 2.115848385 0.
• 103.204 1.961416419 0.
• 100.482 1.826090408 0.
• 97.9577 1.706754777 0.
  • 95.608 1.600905524 0.
• 93.4148 1.506508544 0.
• 91.3619 1.42189996 0.
• 89.4355 1.345710353 0.
  • 87.623 1.276807133 0.
• 85.9134 1.214244603 0.
  • 84.297 1.157228057 0.
• 82.7649 1.105088728 0.
• 82.1737 1.085468292 0.
• 81.5943 1.066503833 0.

3. Finding the distillate and bottom product

0

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

MOLE FRACTION OF BENZENE IN VAPOR PHASE

MOLE FRACTION OF BENZENE IN LIQUID PHASE

F, XF

D, XD

W, XW

P = 1 bar,

F = 80 kmol/hr

XF = 0.

XD = 0.

XW = 0.

Overall mass balance,

100 =D+W ( ) 1

F=D+W

Mass balance for benzene,

80 0.6 D0.95 W*0.04 ( ) 2

F.X F=D.XD+W.XW

D = 48.352 kmol/hr

W = 51.648 kmol/hr

Setting a suitable operating reflux ratio

4. Determination of minimum reflux ratio

4.1. q line equation

q q XF

q

X

q

q

Y

Molarlatentheatofthefeed

Heatrequiredtovaporize 1 moleoffeed q =

Feed is at saturated vapor condition; therefore according to q line equation q value should be

1. Also q-line is passing through the point of (XF, XF)

4.2. Top operating line equation (at minimum reflux ratio condition)

D m

n m

m n X R

X

R

R

Y

1 −

Top operating line is passing through the point of (XD, XD)

In minimum reflux ratio condition the top operating line and q line intersect in the liquid vapor

equilibrium curve.

5. Constructing the top operating line (TOL) for operating reflux ratio

1

1

n n

n n

Y X

Y X

6. Constructing the bottom operating line (BOL) for operating reflux ratio

BOL can be constructed by using intersection between q line and top operating line and the

bottom product composition (XW, XW).

7. Determination of ideal number of stages

From the above graph the number of steps = 7

Therefore the number of ideal stages required = 7 – 1 = 6

(This distillation column is having a total condenser and a partial reboiler)

Feed tray location = 3 (3rd^ tray)

8. Calculation of flowrates

F : Feed (kmol/hr)

D : Distillate (kmol/hr)

W : Bottom product (kmol/hr)

XF : Feed composition of benzene

XD : Distillate product composition of benzene

XW : Bottom product composition of benzene

V : Upward gas flow rate in rectifying section

V' : Upward gas flow rate in stripping section

L : Downward liquid flow rate in rectifying section

L' : Downward liquid flow rate in stripping section

8.1. Material balance for rectifying section

D = 48.352 kmol/hr

W = 51.648 kmol/hr (From previous section)

Density of liquid at 118 °C = 0. 6 *771.51 + 0. 4 *778.61 = 774.35 kg/m^3

Similarly,

Temperature Density of liquid mixture(kg/m^3 )

Density of liquid mixture(kg/m^3 )

118 °C (391 K) 774.35 2. 325

81 °C (354 K) 814.221 2.

131.52°C (404.

K)

9.1. Rectifying section densities

Rectifying section is operating between 81 °C – 118 °C. Therefore the average density values

are,

3

3

. kg/m

ρ

. kg/m

ρ

V

L

9.2. Stripping section densities

Stripping section is operating between 81 °C – 118 °C. Therefore the average density values

are,

3 '

3 '

. kg/m

ρ

kg/m

ρ

V

L

10. Calculation of column diameter

10.1. Diameter of rectifying section

LV

LV

L

V LV

F

F

ρ

ρ

V

L

F

Assume tray spacing = 0. 45 m

K1 = 0 .0 82

The flooding velocity can be calculated from the correlation given below,

1

1

−

U ms

U

ρ

ρ ρ U K

f

f

V

L V f

Dc- Column diameter

Ac- Column cross sectional area

Ad-Down comer area

Aa – Active area

Ah-Holes area

An- Net area

Ua-Actual vapour velocity required through the column

An = Ac - Ad

Assume Ad=12% of Ac.

An = Ac – 0. 12 Ac

An = 0.88 Ac

7 0% flooding condition,

Ua = 0. 7 Uf

Ua = 0. 7 *1.4 4

Ua = 1. 008 m/s

Finding the average molar mass of the mixture

1 0527811 048106165 91576

- Average molar mass of mixture. *.. *.. gmol

mass fraction of benzene

Dc m

Ac m

An m

U ms

U ms

K

a

f

2

2

1

1

1

−

−

So, this satisfies the above condition table.

10.3. Calculation of diameter of stripping section

This is similar to rectifying section.

'

'

LV

LV

L

V LV

F

F

ρ

ρ

V

L

F

Assume tray spacing = 0. 6 m

K1 = 0 .0 9

The flooding velocity can be calculated from the correlation given below,

1

'

' ' 1

−

U ms

U

ρ

ρ ρ U K

f

f

V

L V f

Ua = 0. 7 *1.4 69

Ua = 1. 028 m/s

3 1

1. 432

−

V m s

V

V V M

n

n

n mixture

2

1. 393
2. 028

An m

Ua An

Vn

Dc m

π*Dc

π*Dc Ac

Ac m

. *Ac

An. *Ac

2

2

2

Therefore the column diameter of rectifying section is 1.42 m

11. Calculation of hole diameter and area

Hole size less than 6.5 mm. Entrainment may be greater with larger hole sizes. ( Page No. 568

Coulson & Richardson 6th volume 4th edition).

Therefore hole diameter is considered as 6mm.

12. Number of Holes in a tray at rectifying section

For a plate there is one down comer on the plate and top down comer also affected to bottom

plate. Therefore two down comer areas need to reduce from active area. So,

2

1. 925

Aa m

Aa

Aa Ac Ac

Aa Ac Ad

Assume holes area is 10% of active area;

2

1. 1925

Ah m

Ah

Ah Aa

Total holes area in a tray = 0.1925 m^2

Numberofholeinaplate

5

5 2

2

−

−

. * m

π*. Area of a hole

13. Number of Holes in a tray at stripping section

2

2

Ah Aa m

Aa m

= = =

15. Column efficiency calculation

Here we can use “Van Winkle’s correlation” for calculate to efficiency of column.

Surface tension of liquid at 1 07.92 °C is calculated according to their molar basis of feed.

For the Uv the maximum value Ua value has been taken from both sections.

Assumption,

The surface tension, viscosity, diffusivity values for ethyl benzene could not find for this

particular temperature. So the surface tension, viscosity, diffusivity values of benzene is

assumed as for the mixture.

σL = 17_._ 5526 mN/m

μ. mPa*s

Uv ms

Uv Ua

L^0238112

1

max

−

The average density values were taken for vapor and liquid densities,

3

3

kg m

kg m

avg

avg

V

L

=





Weir height for 1 atm column is 40-90mm. Therefore assume that the weir height of the column

is 50 mm.

. C

Average temperature of column = 

DLK 3. 01 * 10 m / s − 9 2 =

hW = 50 mm

15.1. Fractional area (FA) calculation

columncrosssectionalarea

ofholes

avg

avg

FA

Total

Area FA

15.2. Surface tension number calculation,

Dg = (^) − 3 =

15.3. Liquid Schmidt number calculation,

9

3 = (^) − =

− Sc

15.4. Reynolds number calculation;

Re = (^) − 3 =

Therefore,

1. 014 0. 25 0. 08

mv

mv

mv

E

E

E

Therefore the efficiency of column = 48.7%

16. Calculation of actual number of stages required