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MASS TRANSFER OPERATION II CH 4152

DESIGN OF A BERL SADDLES PACKED DISTILLATION COLUMN

NAME

: N.G.A.K.K.N. NISSANKA

INDEX NO

: 090354 T

DATE OF SUB

: 2013-08-01

Contents INTRODUCTION .................................................................................................................................... 2 Selection of suitable packing material and size ....................................................................................... 3 Material selection ................................................................................................................................. 3 Size selection ........................................................................................................................................ 3 Pressure drop across packing ................................................................................................................... 4 HETP value .............................................................................................................................................. 4 Number of ideal stages required............................................................................................................... 5 Construction of Q line .......................................................................................................................... 6 Construct top operating line (TOL) for minimum reflux ratio ............................................................. 6 Construct top operating line (TOL) for actual reflux ratio ................................................................... 7 The column height .................................................................................................................................... 9 Feed tray location ..................................................................................................................................... 9 Diameter of the column ............................................................................................................................ 9 Flow rates and compositions calculation ............................................................................................ 10 Diameter for rectifying section ........................................................................................................... 11 Diameter for stripping section ............................................................................................................ 15 Heat load of condenser ........................................................................................................................... 19 Heat load of re-boiler ............................................................................................................................. 20 Summary ................................................................................................................................................ 21 References .............................................................................................................................................. 22

1

INTRODUCTION Generally packed columns are used for distillation, gas absorption and liquid-liquid extraction. The gas-liquid contact in a packed column is continuous, not stage-wise, as in a plate column. The liquid flows down in the column over a packing surface and the vapor (or the gas) moves countercurrently, up the column. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution through the packed bed, and this is an important consideration in packed column design. I can arrange given data for distillation column as bellow 

System

: chloroform-benzene



Operating pressure

: 1 bar



Feed rate

: 48 kmol/hr



Feed condition

: 50% sat. liquid and 50% sat. vapour



Feed composition

: 45 mol% chloroform



Distillate composition : 85 mol% chloroform



Bottom product composition : 20 mol% chloroform



Column type

: packed column



Packing type

: berl saddles

As well as we design distillation column with a total condenser and a partial re boiler I need to find bellow data and calculations for this design 

Suitable packing material and packing size



Number of ideal stage required



Column height and diameter



Feed tray location condenser and re boiler heat loads

2

Selection of suitable packing material and size Material selection There two packing method for packed distillation which random and structured packing. According to the given data we need to use berl saddles type packing material. Berl saddles type is used in random packing method. So we have to use random packing method for our distillation process. In berl saddles type has different type raw material such as ceramic, metal, plastic and carbon. Lot of plastic type reacts with chloroform without polythene and polypropylene. As well as some pressure and temperature condition we can ignore plastic type. When we consider the ceramic and plastic both are good for our distillation column. As well as weight of column increase due to ceramic weight higher than metal. So metal is good for packing material. But consider the availability in market most berl saddles type are ceramic. Therefore we use ceramic berl saddle for our packing material. Material

: ceramic berl saddle

Size selection According to market data we can categorize sizes and there data as bellow table

Packing factor Fp 110 98 52 40

When flow rate increase we need to increase packing size. As well as when size increase efficiency decrease. Our flow rate is got low value. So we can use smaller size packing such as (19 - 25) mm. But low sizes get high weight. So I select packing size as a 25 mm. Size

: 25 mm

3

Pressure drop across packing

We need to assume pressure drop across packing using above table. It can be assumed as a 0.75 inch water /ft packing. That means which is equal to the 62.5 mm water /m packing. HETP value

According to our packing size we can get HETP value from below table as 0.46 m.

4

Number of ideal stages required Chloroform-benzene vapour-liquid equilibrium data at 1 bar for 24 data points

temperature(K) 353.26 352.67 352.36 349.80 350.87 348.85 347.87 347.04 346.51 345.58 344.57 343.78 342.77 341.95 341.07 340.02 338.87 338.13 337.49 336.86 336.31 335.53 335.07 334.28

mole fraction of chloroform in liquid phase (X) 0.0000 0.0269 0.0417 0.1855 0.1332 0.2372 0.2948 0.3506 0.3921 0.4427 0.4897 0.5347 0.5781 0.6209 0.6654 0.7181 0.7579 0.7954 0.8278 0.8603 0.8892 0.9308 0.9571 1.0000

mole fraction of chloroform in vapour phase (Y) 0.0000 0.0437 0.0692 0.2581 0.1849 0.3316 0.3980 0.4749 0.5245 0.5714 0.6257 0.6742 0.7164 0.7553 0.7936 0.8257 0.8595 0.8851 0.9057 0.9262 0.9427 0.9648 0.9792 1.0000

Using above data we can draw Chloroform-benzene vapour-liquid equilibrium curve like below graph.

5

Construction of Q line

q = 0.5 Q line equation (

)

X

: mole fraction of chloroform in liquid phase

Y

: mole fraction of chloroform in vapour phase

Xf

: feed composition of chloroform (

) Y = -1 X + 0.90

Construct top operating line (TOL) for minimum reflux ratio According to given data value of distillate composition is 0.85. So we can construct line go through where (0.85, 0.85) and (Q line and equilibrium curve cutting point).

6

Calculate minimum reflux ratio (Rm) Using TOL

Rm

= 2.5417

Calculate R actual using Rm value R actual =Rm × 1.2= 2.5417×1.2 = 3.05 (1.2 is constant vale we can use it between 1.2-1.5) Construct top operating line (TOL) for actual reflux ratio (

Xd

)

: distillate composition (

)

Y n+1 = 0.7531× Xn + 0.21 Construct Bottom operating line (BOL) using given data According to given data value of bottom product composition is 0.20. So we can construct line go through where (0.20, 0.20) and (Q line and TOL curve cutting point). Q line and TOL curve cutting point = (0.394, 0.501) Equation for Bottom operating line

(

)

L'

: Downward liquid flow rate in stripping section

W

: Bottom product (kmol/hr)

Xw

: Bottom product composition of chloroform

Total stages = 14 but it has partial re boiler Number of ideal stages = 13 7

8

The column height

Feed tray location According to stages counting figure feed location should be between six and seven stages from the top and six and seven stages from the bottom. Therefore feed location should be middle of the column

Diameter of the column

XD

XF

G'

L'

XW F

: Feed (kmol/hr)

D

: Distillate (kmol/hr)

W

: Bottom product (kmol/hr)

XF

: Feed

XD

:

Distillate product composition of chloroform

XW

:

bottom product composition of chloroform

composition of chloroform

9

G

: Upward gas flow rate in rectifying section

G'

: Upward gas flow rate in stripping section

L

: Downward liquid flow rate in rectifying section

L'

: Downward liquid flow rate in stripping section

Flow rates and compositions calculation F : 48kmol/hr

XD

: 0.85

XF

XW

: 0.20

: 0.45

Mass balance for the system F=D+W 48 = D + W Mass balance for the chloroform F XF = D XD + W XW 21.6 = 0.85 D + 0.2 W D =18.46 kmol/hr W = 29.53 kmol/hr

G = L + D = 56.3 + 18.46 = 74.76 kmol/hr (

)

(

) L' = 83.22 kmol/hr

G' = L' - W = 83.22 - 29.53 = 53.69 kmol/hr In Rectifying section and Stripping section, from stage to stage composition of G and L are varied, so it is not possible to get Constant Mass flow rate of vapor and Constant Mass flow rate of liquid. So in this calculation, it is better to take an average value of the Mass flow rate across the Rectifying Section and stripping section. Molecular weight of chloroform

= 1395.4 g/mol

Molecular weight of benzene

= 827.49 g/mol

10

Diameter for rectifying section I consider one stage above the feed tray and top stage for get average values in rectifying section.

11

According to above graph data we can get below data DATA

Value

F

48 kmol/hr

D

18.46 kmol/hr

G

56.30 kmol/hr

L

74.76 kmol/hr

temperature

At one stage above the feed tray Chloroform Composition in liquid

0.42

346 K

Chloroform Composition in vapour

0.52

346.5 K

Chloroform Composition in liquid

0.85

337 K

Chloroform Composition in vapour

0.85

339 K

At top stage

Average temperature for liquid phase Average Chloroform Composition in liquid =

0.62

Average Chloroform Composition in vapour =

0.76

Density of the liquid at 342 K Density of chloroform at temp

= 1395.4 Kg/m3

Density of benzene at temp

= 827.49 Kg/m3

Mass fraction of chloroform in liquid (w/w)

=

Mass fraction of benzene in liquid (w/w)

=1-0.7138 = 0.2862

Volume of the chloroform for 1000 kg

=

Volume of benzene for 1000 kg

=

Molar mass of mixture (M)

=

Density of the mixture ( )

=

Liquid Flow rate

(L×M) = 74.76×103.7 = 7752.61 Kg/hr

12

Density of the vapor at 342 K Saturated vapor pressure of chloroform

= 129.27 KPa

Saturated vapor pressure of benzene

= 70.7 KPa

Average Molar fraction of chloroform

= 0.76

Average molar fraction of benzene

= 0.24

Average molecular weight of vapor mixture = Vapor flow rate

(G×M) = 56.30×109.48= 6163.72 Kg/hr Raoult’s Law for vapour phase

Applying Gas Law for find density

Where P = absolute pressure (Pa)

R = universal gas constant 8.314 J/(mol*K)

V = volume m3

m = mass

n = mols of gas

M= molecular weight

T = absolute temperature K

ρ = density

Assumption: all gasses behavior is ideal

Viscosity of the mixture at 342 K Viscosity of chloroform

=

Viscosity of benzene

=

(∑

)

(

)

Find K4 value finding FLV value and using graph

√

√ 13

According to selected pressure drop value of 62.5 K4 can be found = 1.8 At flooding line K4 is equal to the = 4 So percentage of flooding = √ Packing factor (Fp)

= 63.25 % satisfactory

= 98

(

) (

)

(

(

=

√

14

)

)

Diameter for stripping section I consider one stage bellow the feed tray and bottom stage for get average values in rectifying section.

15

According to above graph data we can get below data DATA

Value

F

48 kmol/hr

W

29.53 kmol/hr

G'

53.69 kmol/hr

L'

83.22 kmol/hr

temperature

At one stage bellow the feed tray Chloroform Composition in liquid

0.39

347 K

Chloroform Composition in vapour

0.50

347.5 K

Chloroform Composition in liquid

0.20

350.5 K

Chloroform Composition in vapour

0.20

349.5 K

At bottom stage

Average temperature for liquid phase Average Chloroform Composition in liquid =

0.27

Average Chloroform Composition in vapour =

0.37

Density of the liquid at 348.6 K Density of chloroform at temp

= 1382.04 Kg/m3

Density of benzene at temp

= 820.22 Kg/m3

Mass fraction of chloroform in liquid (w/w)

=

Mass fraction of benzene in liquid (w/w)

=1-0.3611 = 0.6389

Volume of the chloroform for 1000 kg

=

Volume of benzene for 1000 kg

=

Molar mass of mixture (M)

=

Density of the mixture ( )

=

Liquid Flow rate

(L'×M) = 83.22×89.25 = 7427.38 Kg/hr

16

Density of the vapor at 348.6 K Saturated vapor pressure of chloroform

= 157.76 KPa

Saturated vapor pressure of benzene

= 87.62 KPa

Average Molar fraction of chloroform

= 0.37

Average molar fraction of benzene

= 0.63

Average molecular weight of vapor mixture = Vapor flow rate

(G'×M) = 53.69×93.38= 5013.57 Kg/hr Raoult’s Law for vapour phase

Applying Gas Law for find density

Viscosity of the mixture at 348.6 K Viscosity of chloroform

=

Viscosity of benzene

=

(∑

)

(

)

Find K4 value finding FLV value and using graph

√

√

17

According to selected pressure drop value of 62.5 K4 can be found = 1.4 At flooding line K4 is equal to the = 3.2 So percentage of flooding = √ Packing factor (Fp)

= 66.14 % satisfactory

= 98

(

) (

)

(

(

=

√

18

)

)

Heat load of condenser

data

chloroform

benzene

XD

0.85

0.15

Boiling point Tb

335 k

353.1 K

Critical temperature TC

537 K

562 K

Operating temperature T

339 K

339 K

29486.86 J/mol

30462.9 J/mol

Latent heat @ boiling point Lv,b Gas flow rate G

56.30 kmol/hr

Latent heat at temperature T (

)

For chloroform (

) For benzene

( Latent heat of mixture

L=

Condenser heat load

QC

)

19

Heat load of re-boiler

Data

chloroform

benzene

XD

0.85

0.15

Boiling point Tb

335 k

353.1 K

Critical temperature TC

537 K

562 K

Operating temperature T

350.5 K

350.5 K

Latent heat @ boiling point Lv,b

29486.86 J/mol 30462.9 J/mol

Gas flow rate G'

53.69 kmol/hr

Latent heat at temperature T (

)

For chloroform (

) For benzene

( Latent heat of mixture

L=

Condenser heat load

QC

)

20

Summary

According to all calculations of this report we can summaries all important data in below table data

Value or details

System

Chloroform-Benzene

Column type

Packed bed

Packing type

Berl saddle

Packing material

Ceramic

Packing size

25 mm

Pressure drop across packing

62.5 mm water /m

HETP value

0.46 m

Reflux ratio

3.05

Number of ideal stages

13

F feed

48 kmol/hr

D distillate

18.46 kmol/hr

W bottom product

29.53 kmol/hr

G gas flow in rectifying system

56.30 kmol/hr

L liquid flow in rectifying system

74.76 kmol/hr

G' gas flow in stripping system

83.22 kmol/hr

L' liquid flow in stripping system

53.69 kmol/hr

Column height

6m

Feed tray location

Middle point (3 m from bottom)

Diameter of rectifying system

0.62 m

Diameter of stripping system

0.656 m

Heat load of condenser

462.26 kW

Heat load of re-boiler

450.49 kW

21

References

http://ddbonline.ddbst.de/DIPPR105DensityCalculation/DIPPR105CalculationCGI.exe http://ddbonline.ddbst.de/VogelCalculation/VogelCalculationCGI.exe http://ddbonline.ddbst.de/AntoineCalculation/AntoineCalculationCGI.exe http://www.engineeringtoolbox.com/fluids-evaporation-latent-heat-d_147.html http://www.cheric.org/kdb/kdb/hcvle/showvle.php?vleid=3565 from (http://vle-calc.com/index.html) Colson & Richadson chemical engineering volume 6

22