Distillation Column Design (Methanol-Water)

Distillation Column Design Methanol-Water separation

Welahetti W.P.K 080533H

Distillation Column Design 2012 Table of Contents 1

Introduction ....................................................................................................................... 4

2

Theory ................................................................................................................................ 4

3

2.1

Operating Line equation for Top Operating Line (TOP) .............................................. 4

2.2

Operating Line equation for Bottom operating Line (BOP) ........................................ 6

2.3

Equation of q – factor.................................................................................................. 7

Calculation ......................................................................................................................... 7 3.1

Data ............................................................................................................................. 7

3.2

Calculation of amount of distillate and bottom product ............................................ 7

3.3

Equilibrium data of methanol and water mixture ...................................................... 9

3.4

Calculation of Minimum Reflux Ratio ......................................................................... 9

3.4.1

Drawing q line ...................................................................................................... 9

3.4.2

Drawing Top Operating Line (TOP) at minimum reflux ..................................... 10

3.5

Calculation of Operating Reflux Ratio ....................................................................... 11

3.6

Calculation of Actual number of stages required ..................................................... 11

3.7

Calculation of liquid and vapuor amounts in both sections ..................................... 13

3.8

Liquid and Vapour Density calculation...................................................................... 14

3.8.1

Rectifying section densities ............................................................................... 15

3.8.2

Stripping section densities ................................................................................. 15

3.9

Calculation of column diameter ................................................................................ 16

3.9.1

Diameter of rectifying section ........................................................................... 16

3.9.2

Calculation of diameter of stripping section ..................................................... 19

3.10

Calculation of hole diameter and area .................................................................. 20

3.11

Number of Holes in a tray at rectifying section..................................................... 20

3.12

Number of Holes in a tray at stripping section...................................................... 21

3.13

Flow pattern identification and down comer selection ........................................ 21

3.13.1 Flow pattern of rectifying section ...................................................................... 22 3.13.2 Flow pattern of stripping section ....................................................................... 22 3.14

Column efficiency calculation ................................................................................ 23

3.15

Calculation of actual number of stages required .................................................. 25

3.16

Calculation of height of the column ...................................................................... 25

Prasanna Welahetti

Page 2

Distillation Column Design 2012 3.17

Calculation of feed tray location ........................................................................... 26

4

References ....................................................................................................................... 27

5

Summary .......................................................................................................................... 28

Table of Figure 2.1 TOP Line ............................................................................................................................... 4 2.2 Bottom Operating Line ........................................................................................................ 6 3.1 Data ...................................................................................................................................... 7 3.2 Mass balance........................................................................................................................ 8 3.3 Methanol Water Equilibrium Data ...................................................................................... 9 3.4 Temperature Composition Diagram of methanol/water .................................................... 9 3.5 R min Figure ......................................................................................................................... 10 3.6 Vapour Liquid Equilibrium Curve ....................................................................................... 12 3.7 Column 1 ............................................................................................................................ 13 3.8 Feeding Section .................................................................................................................. 14 3.9 Recommended general conditions and dimensions for tray tower .................................. 18 3.10 selection of Liquid-flow arrangement ............................................................................. 21 3.11 Reverse Flow Down Comer .............................................................................................. 22 3.12 Cross flow down comer ................................................................................................... 22 3.13 Van Winkle’s correlation .................................................................................................. 23

Content of Table 3.1 Density data ....................................................................................................................... 14 3.2 Density of Mixture ............................................................................................................. 15 5.1 Summary ............................................................................................................................ 28

Prasanna Welahetti

Page 3

Distillation Column Design 2012 1 Introduction This is continuous distillation with reflux. There are two sections in fractionation column. The upper section is called the rectification section and lower section is called stripping section. Rectifying section enriches with more volatile components and stripping section enriches with less volatile components. 2

Theory

Here we can use the Mc-Cabe-Thiele method, Assumptions;    2.1

Constant molar overflow There is no any chemical reaction There are no any mass generation, mass accumulation and mass generation Operating Line equation for Top Operating Line (TOP)

2.1 TOP Line

Prasanna Welahetti

Page 4

Distillation Column Design 2012 Material balance around the envelope shown: Overall Balance;

Material Balance for More Volatile component (MVC);

Thus we have,

Under constant molar overflow assumption:

The subscripts can be dropped. Thus, the equation simplifies to:

Re-arranging in the form y = f(x), and Introducing Reflux Ratio R = L / D,

Equation 2.1

This is the Operating Line Equation for the rectifying section (Top Operating Line) or TOL in short. Characteristics: Straight Line Equation Slope Intercept

Prasanna Welahetti

,

Page 5


Operating Line equation for Bottom operating Line (BOP)

2.2 Bottom Operating Line

Material balance around the envelope shown: Under constant molar overflow assumption: Overall Balance;

Material Balance for More Volatile component (MVC);

Thus we have,

Equation 2.2

Prasanna Welahetti

Page 6

Distillation Column Design 2012 Equation of q – factor

2.3

3

Calculation

3.1

Data

3.1 Data

3.2 Calculation of amount of distillate and bottom product These two values can calculate using simple material equations. Assumptions 

At steady state there are no any material losses, material accumulation and material generation.

Prasanna Welahetti

Page 7

Distillation Column Design 2012 

There is no any chemical reaction inside the column

3.2 Mass balance

Overall Material balance around the envelope shown:

Material balance for MVC around the envelope shown:

Solving above two equations;

Prasanna Welahetti

Page 8


Equilibrium data of methanol and water mixture

3.3 Methanol Water Equilibrium Data

3.4 Temperature Composition Diagram of methanol/water

3.4

Calculation of Minimum Reflux Ratio

3.4.1 Drawing q line

Feed is at saturated liquid condition; therefore according to q line equation q value should be 1. Therefore,

Prasanna Welahetti

Page 9

Distillation Column Design 2012 3.4.2 Drawing Top Operating Line (TOP) at minimum reflux

At the minimum reflux ratio; maximum numbers of theoretical stages are given. Therefore TOP line should be pass through interception of equilibrium curve and q-line.

3.5 R min Figure

Gradient of TOL

Prasanna Welahetti

Page 10


Calculation of Operating Reflux Ratio

No hard and fast rules can be given for the selection of the design reflux ratio, but for many systems the optimum will lie between 1.2 to 1.5 times the minimum reflux ratios. Assume operating reflux ratio is 1.25 of minimum reflux ratio. Therefore operating Reflux ratio (R);

3.6

Calculation of Actual number of stages required

Slop of the TOL

Therefore TOL equation;

BOL can easily draw with starting (Xw,Xw) to ending intersection of q line and TOL.

Prasanna Welahetti

Page 11

Distillation Column Design 2012

3.6 Vapour Liquid Equilibrium Curve

Prasanna Welahetti

Page 12

Distillation Column Design 2012 According to figure 3.6 vapour- liquid equilibrium curve it can count number of theoretical plates. Total number of stages There are total condenser and partial reboiler. Total condenser is not act as equilibrium stage, but partial reboiler is act as equilibrium stage. So, reboiler stage need reduce from total theoretical stages. Number of theoretical stages required

3.7

Calculation of liquid and vapuor amounts in both sections

3.7 Column 1

Rectifying section

R

Lo D

According to constant molar overflow;

Material balance for above envelop Prasanna Welahetti

Page 13


Stripping Section

3.8 Feeding Section According to given data, feed is at saturated liquid condition therefore all feeding liquids join with stripping section liquid.

3.8 Liquid and Vapour Density calculation Inside temperature of the distillation column is varying point to point. Therefore average temperature was considered for a section.

Position Feed Distillate Residue

Compositions Compositions of Methanol of Methanol Vapour Liquid Temperature 0.68 77˚C 0.35 0.99 65˚C 0.96 0.04 100˚C 0.02

Density of Water(kg/m3) Liquid Vapour

Density of Methanol(kg/m3) Liquid Vapour

973

0.27

742

2.01

980

0.165

752

1.12

958

0.59

714

4.33

3.1 Density data

Prasanna Welahetti

Page 14


Density of liquid at 77˚C =892.15kg/m3 Similarly;

77˚C

Density of liquid mixture kg/m3 892.15

Density of vapour mixture kg/m3 1.45

65˚C

761.12

1.11

100˚C

953.12

0.74

Temperature

3.2 Density of Mixture

3.8.1 Rectifying section densities

Rectifying section is operating in between 65˚C -77˚C. Therefore at the average temperature density values can calculate. Liquid density of rectifying section

L

=

L

=826.6 kg/m3

Vapour density of rectifying section v v

= =1.28 kg/m3

3.8.2 Stripping section densities

Stripping section is operating in between 100˚C -77˚C. Therefore at the average temperature density values can calculate. Similarly; Liquid density of rectifying section

L’

=922.64 kg/m3

Vapour density of rectifying section

v’

=1.095 kg/m3

Prasanna Welahetti

Page 15


3.9

Calculation of column diameter

3.1 Flooding velocity, sieve plates

3.9.1 Diameter of rectifying section

3.2 Liquid Vapour factor

√

Assume tray spacing = 0.45 m According to figure 3.1;

The flooding velocity can be estimated from the correlation given by Fair Prasanna Welahetti

Page 16

Distillation Column Design 2012  L  v v

U f  K1 √

Dc- Column diameter

Ah-Holes area

Ac- Column cross sectional area

An- Net area

Ad-Down comer area

Ua-Actual vapour velocity required through the column

Aa – Active area

Assume Ad=12% of Ac.

Assume 80% flooding condition;

Average molecular weight of the mixture assuming 50% methanol

(When it calculate the average molar weight, feed methanol composition is 35%, assume inside the distillation column 50% methanol in mixture) V n  169 .18  25 /(1  3600 )

V n  1.175

Prasanna Welahetti

m3 / h

m3 / s

Page 17

Distillation Column Design 2012 Vn  Ua An

Therefore according to above derived equations;

Therefore the column diameter of rectifying section is 0.999m 3.9.1.1 Rechecking the assume tray space value

Here we have assumed tray spacing for the column. According to the “Recommended general conditions and dimensions for tray tower” table, it can check. Tower diameter 1 or less 1 to 3 3 to 4 4 to 8

Tray spacing 0.15 0.5 0.6 0.75 0.9

3.9 Recommended general conditions and dimensions for tray tower

According to above “Recommended general conditions and dimensions for tray tower”

table, diameter of the column is correct. Because assume tray space was 0.45m and Prasanna Welahetti

Page 18

Distillation Column Design 2012 received column diameter is 0.999m. Therefore assumed column space is correct(It is in range). 3.9.2 Calculation of diameter of stripping section

This is similar to rectifying section. √

Assume tray spacing = 0.45 m According to figure 3.1;

√

Assume 80% flooding condition;

There is no vapour amount changes in stripping section with compare to rectifying section. Therefore, V m  1.175

m3 / s

Vn  Ua An

Prasanna Welahetti

Page 19

Distillation Column Design 2012 Therefore according to above derived equations;

Therefore stripping section column diameter is 0.837m 3.10 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. 3.11 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,

Substituting to above equation;

Assume holes area is 10% of active area;

Total holes area in a tray

Area of a hole

Number of hole in a plate

Prasanna Welahetti

Page 20

Distillation Column Design 2012 3.12 Number of Holes in a tray at stripping section

All equations are similar to rectifying section calculation;

Assume holes area is 10% of active area;

Number of hole in a plate

3.13 Flow pattern identification and down comer selection The most common flow pattern is cross flow pattern. It can identify using below graph (figure 3.10 selection of liquid flow pattern).

3.10 selection of Liquid-flow arrangement

Prasanna Welahetti

Page 21

Distillation Column Design 2012 3.13.1 Flow pattern of rectifying section

Liquid flow rate of rectifying section

Column Diameter Therefore the flow pattern is “Reverse flow”(According to figure 3.10)

3.11 Reverse Flow Down Comer

3.13.2 Flow pattern of stripping section

Liquid flow rate of stripping section

Column Diameter Therefore the flow pattern is “Cross flow”. (According to figure 3.10)

3.12 Cross flow down comer

Prasanna Welahetti

Page 22

Distillation Column Design 2012 3.14 Column efficiency calculation

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

3.13 Van Winkle’s correlation

Maximum and minimum temperatures of inside the column are respectively 100C˚ and 65C˚. Assume liquid temperature is average temperature of column. Assume properties of liquid mixture are equal to water properties. Average temperature of column

Surface tension of liquid at 82.5

As the

is calculated according to their molar basis of feed.

; I have taken maximum Ua value from both sections.

Prasanna Welahetti

Page 23

Distillation Column Design 2012 As vapour and liquid densities; average densities were taken,

 

Weir height for 1 atm column is 40-90mm. Therefore here I’m assuming weir height of the column is 45mm.

Fractional area calculation;

Surface tension number calculation;

Liquid Schmidt number calculation;



Reynolds number calculation;

Prasanna Welahetti

Page 24


Therefore;

Therefore efficiency of column

3.15 Calculation of actual number of stages required

I have already calculated required number of theoretical plates. Number of theoretical stages required In other way below equation can use for efficiency calculation;

Therefore number of actual stages required

=17

3.16 Calculation of height of the column

No. of plates Tray spacing Hs = 0.45 m ∆H= 0.5 meter each for liquid hold up and vapor disengagement Prasanna Welahetti

Page 25

Distillation Column Design 2012 ∆H=1 m Assume thickness of a plate is 5mm. Total thickness of trays = 0.085m Height of column

=

Therefore the height of the column is 8.234m.

3.17 Calculation of feed tray location

Assume here plate numbers are count from top to bottom. According to figure 3.6 vapour-liquid equilibrium curve; it can find the feed tray location. Feed composition of methanol is 0.35. This value is at 10th plate (theoretical) of the column. To calculate the actual feed tray location this value need to divide with column efficiency. Therefore feed tray location

Prasanna Welahetti

Page 26

Distillation Column Design 2012 4

References  Separation Process En gineering,2nd Edition, Phillip C.Wankat  Chemical Engineering volume 6, 4th Edition, Coulson & Richardson’s  Mass-transfer Operations,3rd Edition, Robert E.Treybal  http://pubs.acs.org/doi/abs/10.1021/je00019a016  http://www.engineeringtoolbox.com/water-dynamic-kinematic-viscosity-d_596.html  http://www.engineeringtoolbox.com/methanol-properties-d_1209.html

Prasanna Welahetti

Page 27

Distillation Column Design 2012 5

Summary

Name

Sign

Result

Distillate flow rate

D

87.77 kmol/hr

Waste flow rate

W

162.23 kmol/hr

q value

q

1

Minimum Reflux ratio

0.742

Operating Reflux ratio

R

Number of theoretical plates

13

Liquid flow rate of rectifying section

L0,Ln

81.41 kmol/hr

Dc

0.999m

Column Diameter of Stripping Section

Dc

0.837m

Tray Space of both sections

lt

0.45m

Hole diameter

Dh

6mm

Vapour flow rate of rectifying section Flooding velocity of rectifying section Column Diameter of rectifying Section Flooding velocity of Stripping section

Number of hole in a plate at rectifying Section

2106

Number of hole in a plate at Stripping Section Down comer type of rectifying section

1478 Reverse flow down comer

Down comer type of Stripping section

Cross flow down comer

Column efficiency

Emv

Number of actual plates

79% 17

Height of the column 13th plate

Feed tray location

5.1 Summary

Prasanna Welahetti

Page 28

Distillation Column Design (Methanol-Water)

Recommend Documents