Distillation Column Design

(1)

DESIGN OF A PACKED

DISTILLATION COLUMN

NAME : Abeyrathna D.K.M.R.P INDEX NO : 100007E

DATE OF SUB : 04/08/2014 Ethanol and Butyl Acetate mixture

(2)

i

ii

List of Figures

Figure 3.1 Rectifying section ... 6

Figure 3.2 Stripping section ... 9

List of Graphs Graph 3.1 Composition graph of Ethanol ... 7

Graph 3.2 Equilibrium curve of Ethanol Butyl acetate mixture ... 8

Graph 3.3 Temperature-composition diagram of ethanol ... 13

Graph 3.4 Composition marked Temperature-composition diagram of ethanol ... 14

(4)

iii

TERMINOLOGY

x mole fraction of heptane in liquid phase (With suffixes F-feed, D-distillate, W-Bottom product) y mole fraction of heptane in vapor phase (With suffixes F-feed, D-distillate, W-Bottom product) Rmin Minimum reflux ratio

R Operating reflux ratio

yn Mole fraction of heptane in vapor stream leaving upwards the nth theoretical stage

xn Mole fraction of heptane in liquid stream leaving downwards the nth theoretical stage

D Distillate flow rate (kmol/hr) F Feed flow rate (kmol/hr)

W Bottom product flow rate (kmol/hr)

L Downwards liquid stream flow rate in rectifying section (kmol/hr) G Upwards vapor stream flow rate in rectifying section (kmol/hr) L’ Downwards liquid stream flow rate in stripping section (kmol/hr) G’ Upwards vapor stream flow rate in stripping section (kmol/hr) Lw* Liquid mass flow rate along the column (kg/hr)

Vw* Vapor mass flow rate along the column (kg/hr)

ρv Vapor density (kg/m3)

ρL Liquid density (kg/m3)

Fp Packing factor (m-1)

µL Kinematic viscosity of liquid (Ns/m2)

λc Latent heat of condensing vapor at condensor (kJ/kg) λL’ Latent heat of evaporating liquid at reboiler (kJ/kg)

Qc Condensor heat load QR Reboiler heat load

(5)

1

1 PROBLEM

Design a distillation column with a total condenser and a partial reboiler for the following separation. Data:

System : Ethanol- butyl acetate Operating pressure :1 bar

Feed rate : 130kmol/hr Feed condition : saturated liquid Feed composition : 62mol% ethanol Distillate composition : 97mol% ethanol Bottom product composition : 5mol% ethanol Column type : Packed column Packing type : Intalox saddles

Select a suitable packing material, packing size, pressure drop across packing or percentage of flooding and a reflux ratio. Obtain an appropriate HETP value HTU from the literature and determine the Number of ideal stages required the column height and the diameter of the column, feed tray

location and the Condenser and Reboiler heat loads.

The report should include detail calculation, graphical constructions, data used for the calculation, assumptions made and the references. Mechanical design and construction are not required. Mc-Cabe and Thiele method, Ponchan - Savarit method or HTU-NTU method can be used. All the important information should be summarized in the last page of the report.

(6)

2

2 DESIGN CALCULATION

2.1 Packing Material

The distillation is supposed to be done at 1 bar pressure.The packing is selected as pressure drop is reduced.The pressure drop has to be maintained to keep the column below the flooding condition. There two packing method for packed distillation which random and structured packing. According to the given data we need to use Intalox saddles type packing material. Intalox saddles type is used in random packing method. So we have to use random packing method for our distillation process. In intalox saddles type has different type raw material such as ceramic, metal, plastic and carbon. Many plastic type reacts with butyl acetate. As well as some temperature condition are affects for plastic. There for we can ignore plastic type. Weight of column increase due to ceramic. Weight of ceramic is higher than metal. So metal is good for packing material. Some metal saddles are reacted with ethanol and butyl acetate. Stainless steel, Hastelloy saddles can be used. But that material cost is very high and consider the availability in market most intalox saddles types are ceramic. Therefore use ceramic intalox saddle for packing material.

Material : ceramic intalox saddle

Table 2.1 Details of selected packing

Name Intalox Saddles

Material Ceremic

Size (mm) 38

Bulk density (kg/m3) 625 Specific Surface area (a) (m2/m3) 194 Packing Factor (Fp)(m-1) 170

HETP 0.60 - 0.75

(7)

(8)

4

2.2 Vapour pressure of gas

Saturated vapour pressure can be calculated using below equation. Limits of this equations can be use;

For Ethanol: 159.05K to 514K

For Butyl Acetate: 199.65K to 574.5K [3]

Component C1 C2 C3 C4 C5

Ethanol 122.82 -9253.2 -14.99 1.05E-05 2

Butyl Acetate 73.304 -7122.3 -7.1424 2.89E-06 2

Table 2.3 Saturated pressure of Ethanol & Butyl acetate in different temperatures

Temperature(0C) Vapour Pressure(mmHg)

Ethanol Butyl acetate

78.37 760 151 80 807 161 85 978 195 90 1177 235 95 1408 281 100 1675 334 105 1982 395 110 2334 465 115 2733 544 120 3186 634 125 3698 735 126 3806 760

From Raoult’s law

Applying Raoult’s law for Ethanol & Butyl acetate;

From Daltons Law;

(9)

5 Therefore,

-

- -

From Dalton’s law;

(

⁄

)

Values of and can be calculated for each temperature using equation

Consider the vapour pressure values at 800C.

,

- ⁄_-

( ⁄ )

Table 2.4 Equilibrium data for composition

Temperature(0C) Vapour Pressure(mmHg) Mole Fraction Ethanol Butyl acetate

X

Ethanol

Y

Ethanol

78.37 760 151 1.00 1.00 80 807 161 0.93 0.98 85 978 195 0.72 0.93 90 1177 235 0.56 0.86 95 1408 281 0.42 0.79 100 1675 334 0.32 0.70 105 1982 395 0.23 0.60 110 2334 465 0.16 0.48 115 2733 544 0.10 0.35 120 3186 634 0.05 0.21 125 3698 735 0.01 0.04 126 3806 760 0.00 0.00

(10)

6

2.3 Construction of Q line

Feed condition is saturated liquid.

2.4 Mass balance for rectification section

Assumption

• Constant molar overflow • Reflux is at its boiling point

Material balance for the system boundry

V = L + D---(1) For m.v.c.( Ethanol)

V

L

(11)

7

V. yn+1 = L. xn + D. xD ---(2)

From (1) and (2)

Operating line equation for the rectification section (Top Operating Line)







1 R



.x

(

)

1 .x

R

1 R

y

_n ₁ _n _D



A







Gradient of the operating line = (R/(R+1)) Intercept = xD/(R+1)

Operating line intersects, y = x line at ( xD, xD)

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 M o le f rac ti o n o f E th an o l i n v ap o u r

Mole fraction of Ethanol in Liquid

q line

top operating line





n





D 1 n .x D L D .x D L L y     

(12)

8 Top operating line equation in minimum reflux



_

_

_

_

D m in n m in m in 1 n

.x

R

1

1 .x

R

1 y







R

Ractual is in between 1.2Rmin and 1.5Rmin.

Let assume Ractual = 1.3×Rmin=1.3×0.2963=0.3852

=>









D actual n actual 1 n

.x

R

1

1 .x

R

1 y





 actual

R

00

7 .

0 .x

278 .

0 y

_n_₁



_n



0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 A xi s Ti tl e Axis Title

(13)

9

Number of Theoretical plates required for the separation = No. of steps – 1 =7-1

= 6

2.5 Mass balance for stripping section

Material balance for the system boundary;

For the m.v.c. (Ethanol)

Bottom Operating Line

(B)

m

.x

W

L

W

m

.x

W

L

1 m

y

_



_

_





_

_



From graph

(14)

10 Gradient=

(15)

11

2.6 Flow rates and compositions calculation

F : 130kmol/hr XF : 0.62

XD : 0.97

XW : 0.05

Mass balance for the system

Mass balance for ethanol

From (1) & (2) From bottom operating line;

(16)

(17)

13 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Bo ili n g p o in t( C ) Mole fraction Ethanol in liquid Ethanol in Vapor

(18)

14 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Bo ili n g p o in t( C) Mole fraction Ethanol in liquid Ethanol in Vapor

(19)

15

2.7 Rectifying Section

The composition of the gas and liquid streams vary along the column and so the mass flow rates and the stream densities for gas and liquid also vay along the column. Therefore for the rectifying section, the average composition of the feed and the distillate has been considered to determine the mass flow rates and the densities.

Ethanol molar fraction at feed = 0.62

Ethanol molar fraction at distillate = 0.97 Molar mass of Ethanol= 46.06844 kg/kmol Molar mass of Butyl Acetate=116.16 kg/kmol

Average temperature value of rectifying section =

Average composition for rectifying section taken at as X= 0.780 and Y = 0.945 Molar mass in feed=

Molar mass in Distillate Average molar mass

Mass fraction of A= Mass fraction of Ethanol MEthanol =

116 16 = 0.5844

Mass fraction of Butyl acetate MButyl acetate = 1- MEthanol

= 1-0.5844 = 0.4156

Mass fraction of Ethanol MEthanol =

116 16 = 0.8720

Mass fraction of Butyl acetate MButyl acetate = 1- MEthanol

= 1-0.8720 = 0.128

2.7.1 Density Calculation

Density of organic compound

[2]

(20)

16

Table 2.5 Density calculation data Constants Ethanol Butyl acetate

C1 1.6288 0.67794 C2 0.27469 0.2637 C3 514 575.4 C4 0.23178 0.29318 Temperature(0C) Densities(mol/dm3 ) 83.5 15.835 7.017 104 15.348 6.819 Density of Ethanol at = 729.4937 Kg.m-3

Density of Butyl acetate at = 815.0947Kg.m-3

For Vapour region

By Dolton’s law

Where, is partial pressure of ethanol

To calculate density of ethanol at this temperature and pressure, we can calculate as follow

116 16

(21)

17

Mass flow rate of vapor stream = V kmol/hr x Average molar mass kg/kmol

= 6742.8575 Mass flow rate of liquid stream = L kmol/hr x Average molar mass kg/kmol

= 1875.0641

2.7.2 Viscosity Calculation

Liquid Viscosity can be represent by function of temperature. Where C1, C2, C3, C4 are constant and T is

temperature. [3]

Table 2.6 Viscosity data

Constants Ethanol Butyl acetate

C1 7.875 -17.488 C2 781.98 1478.2 C3 -3.0418 0.91828 C4 - - C5 - - Temperature(0C) Viscosity(Pa.s) 83.5 4.072*10-4 3.5427*10-4 104 3.049*10-4 2.9765*10-4

Table 2.7 Parameter values of rectifying section

Average molar mass

6742.8575 1875.0641

(22)

18

Graph 2.5 Flooding curve

The pressure drop inside the packed column determines the maximum possible gas velocity through the packed column. There is a maximum value for the gas velocity at a given pressure drop which further increment of gas velocity causes flooding. So the safety margin we provide to save the column from flooding determines the pressure drop we maintain within the column. For a selected pressure drop, the correlation to determine the internal gas velocity are illustrated in the figure below. Now the above calculated internal liquid and gas flow rates can be used to determine the allowed gas phase velocity that inside the column without flooding.

(23)

19

for the rectifying section, √ => _{1875 0641} √

FLV = 0.1195

For random packing the pressure drop will not normally exceed 80 mm of water per meter of packing height. At this value the gas velocity will be about 80 per cent of the ﬂooding velocity (Sinnott, 2005). However the recommended pressure drop for packed column distillation to keep in a safer region from flooding is between 40-80 mm of water column per meter of packing height (Sinnott, 2005). So let us consider 60 mm water column pressure drop per meter of packed height for our design.

Therefore using the pressure correlation closer to 60 mm water/m in Graph 3.5 above, the relevant K4 value for the rectifying section is 1.8.

:.

But the total mass flow rate in vapor stream of the rectifying section is supposed to be 6742.8575 kg/hr according to the above calculation.

So if the column diameter is D,

Column cross section area

Drectifying = 1.6827 m

2.8 Stripping Section

Similarly as above, the average composition of the feed and the residue has been considered to determine the mass flow rates and the densities of vapor and liquid streams of the stripping section. Ethanol molar fraction at feed = 0.62

Ethanol molar fraction at distillate = 0.05

Therefore average molar fraction of Ethanol in rectifying section = (0.62 + 0.05)/2 = 0.335 Molar mass in feed=

Molar mass in Distillate Average molar mass

Average temperature value of stripping section =

(24)

20 Mass fraction of Ethanol in liquid MEthanol =

116 16 = 0.13549

Mass fraction of Butyl acetate in liquid MButyl acetate = 1- MEthanol

= 1-0.13549 = 0.86451 Mass fraction of Ethanol in vapour MEthanol =

116 16 = 0.4676

Mass fraction of Butyl acetate in vapour MButyl acetate = 1- MEthanol

= 1-0.4676 = 0.5324 Density of Ethanol at =707.0584 Kg.m-3

Density of Butyl acetate at = 792.095 Kg.m-3

116 16

Mass flow rate of vapor stream = V’ kmol/hr x Average molar mass kg/kmol

= 12732.2822 Mass flow rate of liquid stream = L’ kmol/hr x Average molar mass kg/kmol

(25)

21

Table 2.8 Parameters values of stripping section

Average molar mass

12732.2822 17315.9224 for the rectifying section,

_√

=>

17315 9224

√

FLV = 0.0279

Similarly as in rectifying section, a pressure drop of around 60mm water column per meter has been assumed. Therefore using the pressure correlation closer to 60 mm water/m in figure 3 above, the relevant K4 value for the rectifying section is 2.9.

:.

But the total mass flow rate in vapor stream of the rectifying section is supposed to be 12732.2822 kg/hr according to the above calculation.

So if the column diameter is D,

Column cross section area

DStripping = 1.5446 m

2.9 The column height

2.10 Feed tray location

According to stages counting figure feed location should be between two & three stages from the top. Feed tray location=

(26)

22

2.11 Heat load

2.11.1 Condenser heat load

The total condensor is used in the vapor stream leaving the top most column to totally convert it into liquid form. So the resultant liquid will consist of the same composition as the vapor which was subjected to condensation and the condensor heat load will be the latent heat required to condense this vapor stream up to its’ dew point.

Flow rate of the vapor stream of the rectifying section (G) = kmol/hr As we have 97% Ethanol and 3% butyl acetate in this stream,

Component (kJ/kmol) (K) (K)

Ethanol 38600 514 351.37

Butyl acetate 35893.44 575.4 399

So latent heat of the Ethanol (λ1) = 38842.668 kJ/kmol

latent heat of the Butyl acetate (λ2) = 39264.243kJ/kmol

Average latent heat (λ) = 38842 668 39264 243 Therefore condensor heat load (Qc) = V x λ

= =4335009.784kJ/hr

= =1202.78kW

2.11.2 Reboiler heat load

Component (kJ/kmol) (K) (K)

Ethanol 38600 514 351.37

Butyl acetate 35893.44 575.4 399

So latent heat of the Ethanol (λ1) = 34765.765 kJ/kmol

(27)

23

Average latent heat (λ) = 3 Therefore Reboiler heat load (QR) = V' x λ

= =4983215.651kJ/hr

= =1384.167kW

(28)

24

3

SUMMARY

 Column type =Packed column

 Packing material = 38 mm ceremic Intalox Saddles

 Pressure drop across packing=60mm H2O/m  Reflux ratio =0.3852

 Number of theoretical plates =6

 Column Height =

 Column diameter = 1.6827 m m for rectifying section = 1.5446 m m for stripping section

 Feed entering location=

 Condensor heat load = 1202.78kW

(29)

25

4 REFERENCES

[1] R. H. Don W.Green, "Vapor Pressure of Inorganic and Organic Liquids," in Perry's Chemical

Engineers' Hand Book, The Mc-Graw-Hill companies, 2008, pp. 2-56 to 2-60.

[2] R. H. P. Don W. Green, "Densities of Inorganic and Organic Liquids," in Perry's Chemical

Engineers' Handbook, Eighth Edition, MC-Graw Hill, 2008, pp. 2-98 and 2-100.

[3] R. H. P. Don W. Green, "Viscosity of Inorganic and Organic Liquids," in Perry's Chemical

Engineers' Handbook, Eighth Edition, Mc-Graw Hill, 2008, pp. 2-427 and 2-429.

[4] R. Sinnott, Coulson & Richardsons' Chemical Engineering Series-Chemical Engineering Design, Oxford: Elsevier Butterworth-Heinemann, 2005.

[5] S. B. Thakore and B. Bhatt, Introduction to Process Engineering and Design, New Delhi: Tata McGraw-Hill Education, 2007.

[6] L. AceChemPack Tower Packing Co., "Random & Structured Ceramic Packing," 2000. [Online]. Available: http://www.tower-packing.com/Dir_ceramic_packing.htm.

Distillation Column Design

DESIGN OF A PACKED