DESIGN OF A PACKED DISTILLATION COLUMN Ethanol and Butyl Acetate mixture
NAME
: Abeyrathna D.K.M.R.P
INDEX NO
: 100007E
DATE OF SUB : 04/08/2014
CONTENTS TERMINOLOGY.................................................................................................................................... iii 1
PROBLEM ........................................................................................................................................ 1
2
DESIGN CALCULATION .............................................................................................................. 2 2.1
Packing Material ........................................................................................................................ 2
2.2
Vapour pressure of gas .............................................................................................................. 4
2.3
Construction of Q line ............................................................................................................... 6
2.4
Mass balance for rectification section ....................................................................................... 6
2.5
Mass balance for stripping section ............................................................................................ 9
2.6
Flow rates and compositions calculation ................................................................................. 11
2.7
Rectifying Section ................................................................................................................... 15
2.7.1
Density Calculation .......................................................................................................... 15
2.7.2
Viscosity Calculation ....................................................................................................... 17
2.8
Stripping Section ..................................................................................................................... 19
2.9
The column height ................................................................................................................... 21
2.10 Feed tray location .................................................................................................................... 21 2.11 Heat load .................................................................................................................................. 22 2.11.1
Condenser heat load ......................................................................................................... 22
2.11.2
Reboiler heat load............................................................................................................. 22
3
SUMMARY .................................................................................................................................... 24
4
REFERENCES ............................................................................................................................... 25
List of Table Table 3.1 Details of selected packing....................................................................................................... 2 Table 3.2 Details of packing materials ..................................................................................................... 2 Table 3.3 Saturated pressure of Ethanol & Butyl acetate in different temperatures ................................ 4 Table 3.4 Equilibrium data for composition ............................................................................................ 5 Table 3.5 Density calculation data ......................................................................................................... 16 Table 3.6 Viscosity data ......................................................................................................................... 17 Table 3.7 Parameter values of rectifying section ................................................................................... 17 Table 3.8 Parameters values of stripping section ................................................................................... 21
i
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 Graph 3.5 Flooding curve....................................................................................................................... 18
ii
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
iii
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.
1
2 2.1
DESIGN CALCULATION 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 Size (mm) Bulk density (kg/m3) Specific Surface area (a) (m2/m3) Packing Factor (Fp)(m-1) HETP
Ceremic 38 625 194 170 0.60 - 0.75
Table 2.2 Details of packing materials
2
3
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
Component Ethanol Butyl Acetate
C1 122.82 73.304
[3]
C2 -9253.2 -7122.3
C3 -14.99 -7.1424
C4 1.05E-05 2.89E-06
C5 2 2
Table 2.3 Saturated pressure of Ethanol & Butyl acetate in different temperatures Temperature(0C) 78.37 80 85 90 95 100 105 110 115 120 125 126
Vapour Pressure(mmHg) Ethanol Butyl acetate 760 151 807 161 978 195 1177 235 1408 281 1675 334 1982 395 2334 465 2733 544 3186 634 3698 735 3806 760
From Raoult’s law
Applying Raoult’s law for Ethanol & Butyl acetate;
From Daltons Law;
In the mixture, only contain Ethanol and Butyl Alcohol
4
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) 78.37 80 85 90 95 100 105 110 115 120 125 126
Vapour Pressure(mmHg) Ethanol Butyl acetate 760 151 807 161 978 195 1177 235 1408 281 1675 334 1982 395 2334 465 2733 544 3186 634 3698 735 3806 760
5
Mole Fraction
XEthanol 1.00 0.93 0.72 0.56 0.42 0.32 0.23 0.16 0.10 0.05 0.01 0.00
YEthanol 1.00 0.98 0.93 0.86 0.79 0.70 0.60 0.48 0.35 0.21 0.04 0.00
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
V
L
Figure 2.1 Rectifying section
Material balance for the system boundry V = L + D-------------------------------------(1) For m.v.c.( Ethanol) 6
V . yn+1 = L. xn + D. xD ---------------------------(2) From (1) and (2)
y n 1
L
L
D
.x n
D L D.x D
Operating line equation for the rectification section (Top Operating Line)
y n 1
R 1 .x n .x ( A) 1 R 1 R D
Gradient of the operating line = (R/(R+1)) Intercept
= xD/(R+1)
Operating line intersects,
y = x line at ( xD, xD)
1.00 0.90
Mole fraction of Ethanol in vapour
0.80 0.70 0.60 q line 0.50 top operating line 0.40 0.30 0.20 0.10 0.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
Mole fraction of Ethanol in Liquid
Graph 2.1 Composition graph of Ethanol
7
1.00
Top operating line equation in minimum reflux
y n 1
Rm in 1 .x n .x 1 R m in 1 R m in D
Ractual is in between 1.2Rmin and 1.5Rmin. Let assume Ractual = 1.3×Rmin=1.3×0.2963=0.3852 =>
y n 1
Ractual 1 .x n .x 1 R actual 1 R actual D
y n 1 0.278 .x n 0.700 1.00 0.90 0.80 0.70
Axis Title
0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Axis Title
Graph 2.2 Equilibrium curve of Ethanol Butyl acetate mixture 8
Number of Theoretical plates required for the separation = No. of steps – 1 =7-1 =6 2.5
Mass balance for stripping section
Figure 2.2 Stripping section
Material balance for the system boundary;
For the m.v.c. (Ethanol)
Bottom Operating Line
y m1
L W .x m .x (B) LW LW m
From graph 9
Gradient=
10
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;
11
12
130
Ethanol in liquid
129 128
Ethanol in Vapor
127 126 125 124 123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 108 107
Boiling point(C)
106 105 104 103 102 101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76 75 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Mole fraction
Graph 2.3 Temperature-composition diagram of ethanol
13
0.8
0.9
1.0
130 129 128
Ethanol in liquid
127
Ethanol in Vapor
126 125 124 123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 108 107
Boiling point(C)
106 105 104 103 102 101 100 99 98 97 96 95 94 93 92 91 90 89 88 87 86 85 84 83 82 81 80 79 78 77 76 75 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Mole fraction
Graph 2.4 Composition marked Temperature-composition diagram of ethanol 14
1.0
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
Mass fraction of Butyl acetate MButyl acetate = 1- MEthanol = 1-0.8720 = 0.128
2.7.1 Density Calculation Density of organic compound
[2]
Where C1, C2, C3, C4 are constant and T is temperature.
15
= 0.8720
Table 2.5 Density calculation data Constants C1 C2 C3 C4 Temperature(0C) 83.5 104
Ethanol Butyl acetate 1.6288 0.67794 0.27469 0.2637 514 575.4 0.23178 0.29318 Densities(mol/dm3 ) 15.835 7.017 15.348 6.819
= 729.4937 Kg.m-3
Density of Ethanol at
= 815.0947Kg.m-3
Density of Butyl acetate at
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
16
Mass flow rate of vapor stream
= V kmol/hr x Average molar mass kg/kmol =
Mass flow rate of liquid stream
6742.8575
= 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 C1 C2 C3 C4 C5 Temperature(0C) 83.5 104
Ethanol 7.875 781.98 -3.0418 Viscosity(Pa.s) 4.072*10-4 3.049*10-4
Butyl acetate -17.488 1478.2 0.91828 3.5427*10-4 2.9765*10-4
Table 2.7 Parameter values of rectifying section
Average molar mass
6742.8575 1875.0641
17
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.
Graph 2.5 Flooding curve
18
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 = Average composition for stripping section taken at 19
as X= 0.245 and Y= 0.620
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 = 792.095 Kg.m-3
Density of Butyl acetate at
116 16
Mass flow rate of vapor stream
= V’ kmol/hr x Average molar mass kg/kmol =
Mass flow rate of liquid stream
12732.2822
= L’ kmol/hr x Average molar mass kg/kmol =
17315.9224
20
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=
21
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 Ethanol Butyl acetate
(K) 514 575.4
(kJ/kmol) 38600 35893.44
(K) 351.37 399
So latent heat of the Ethanol (λ1) =
38842.668 kJ/kmol
latent heat of the Butyl acetate (λ2) = Average latent heat (λ)
39264.243kJ/kmol
=
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) Ethanol 38600 Butyl acetate 35893.44
(K) 514 575.4
So latent heat of the Ethanol (λ1) =
(K) 351.37 399 34765.765 kJ/kmol
latent heat of the Butyl acetate (λ2) =
3kJ/kmol 22
Average latent heat (λ) Therefore Reboiler heat load (QR)
=
3 = V' x λ = =4983215.651kJ/hr = =1384.167kW
23
3
SUMMARY
Column type
=Packed column
Packing material
= 38 mm ceremic Intalox Saddles
Pressure drop across packing=60mm H2O/m
Reflux ratio
Number of theoretical plates =6
Column Height
=
Column diameter
= 1.6827 m m for rectifying section
=0.3852
= 1.5446 m m for stripping section
Feed entering location=
Condensor heat load = 1202.78kW
Reboiler heat load
= 1384.167kW
24
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.
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