6.9
DISTILLATION COLUMN (DC-101)
(Designed by MUHAMMAD SYAHMIE, A154282)
The distillation column is used to separate xylene from the product stream which
contains water. Water is light key component (LK) will vaporizing toward the distillate
part while Xylene act as the heavy key component (HK).
Figure 6.11
Illustration for distillation column, DC-101
Number of Stages
Number of equilibrium stages within the distillation column can be determine by using
Short –Cut method. By using Antoine Equation, vapor pressure can be obtained.
Table 6.17 Antoine equation for Xylene and Water
Component and Its Equation
Water
log 𝑃𝑠𝑎𝑡 = 5.109459 −
1678.948
𝑇(℃) + 228.97
Xylene
log 𝑃𝑠𝑎𝑡 = 4.13785 −
1465.39
𝑇(℃) + 215.512
The K-value can be obtained using Roult,s Law where 𝑲𝒊 =
KD,B
Relative volatility, α(i,j) = K
D,T
𝑷𝒔𝒂𝒕
𝑷
α(i,j) =
KB,B
KB,T
Table 6.18 K-value and relative volatility for xylene and water
Component
Water
Xylene
K i,D
0.001911
0.000577
α,D
3.313767
3.313767
Ki,B
0.001911
0.000577
Mean relative volatility, αm = (3.313767 x 3.313767)0.5 = 3.313767
Hence,by using Fenske equation:
N min 


log xi , N 1  / xi ,1  x j ,1 / x j , N 1 
log  i , j
Nmin = 7 equilibrium stages
Rmin is determined using the Underwood equations.
From First Underwood equation,
Liquid, q=1
1 q  
  z
   
i ,r 
i ,r
i, f

θ = 2.084
From the Second Underwood equation,
Rmin = 1.941497
Where R=2Rmin = 3.882994
By using Gilliland correlation equation,
X = 0.3976038
α,B
3.313767
3.313767
N = 12 stages
Hence the actual number of equilibrium stages is 11 plus a partial reboiler
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 =
=
𝑁𝑚𝑖𝑛
𝑁−1
7
× 100%
12
= 58.33%
Hence, the tray efficiency is 58.33 %.
Diameter of Distillation Column
Parameter that are related in order to determine the diameter of distillation column are:
Tray spacing, ts = 24 inches = 0.6096 m
σ = liquid surface tension = 72 dynes/cm
w = liquid flow rate = 44.576 kg/h
W= vapour flow rate = 255.124 kg/h
ρL = liquid density =860 kg/m3
ρv = vapour density = 997 kg/m3
Mv = 18 kg/kmol
V = 0.07kmole/h
f = 0.8
By using formula below;
𝜌𝐿−𝜌𝑣
Uv= (−0.171𝑡𝑠 2 + 0.271𝑡𝑠 − 0.047)( 𝜌𝑣 )0.5
Uv = 1.97318284
The diameter of the DC is;
4Vw
Dc = √𝜋𝜌𝑣𝑈𝑣
Where, Vw = Max vapor rate in kg/s
Dc = 1.203371373 m
Therefore, the diameter of the column is 1.2 m whichthe diameter normally in between
1.52 m to 4.57 m for industrial distillation column with plate tray.
Height of Distillation Column
Height of absorption tower, Ho = Tray spacing x Number of Stages
= 1.2 x 5
=6m
Ratio of the height and diameter is, Ho/DT = 5
In industrial, the Ho/DT ratio is always less than 20-30. Thus, the distillation column
structure is reasonable.
Tray Selection
Tray distillation column is chosen rather than packed distillation column because it
can be used in high liquid rate composition feed and temperature variable. Besides, it
is easy to predict the performance and have large diameter column. The type of tray
that is chosen is sieve-tray. Sieve trays are flat perforated plate in which vapor rises
through small holes in tray floor while bubbles through liquid in fairly uniform
manner. Sieve-tray have simple construction, low maintenance cost and low fouling
tendency.
Table 6.18 shows the summary calculation for distillation column.
Table 6.19
Parameter
Diameter, D
Height, h
Number of Stages, N
Summary for distillation column
Value
1.2 m
6m
12
6.12
REBOILER (R-101)
(Designed by Muhammad Syahmie Bin Sobri, A154282)
Reboiler are used with the distillation columns to vaporize a fraction of the bottom
products which is xylene, whereas in a vaporizer essentially all the feed is vaporized.
Type of reboiler chosen is thermosyphon reboiler because it less fouling, less
maintenance cost, less space and piping required, and high heat transfer rates thus less
power is used during distillation.
From journal Researchgate (BA Bhanvase, M P Deosarkar, S RShirsath& RWGaikwad,
June 2017),
Properties of xylene at 1 bar
Boiling point, TB = 132 °C
Specific heat of Xylene = 1.72 kj/kmol
Critical pressure, Pc = 35.2 bar
Heat loads. Q = 806.52 kW
Operating pressure, Po = 1 bar
Steam saturation temperature at 5 bar (°C)
Tsat = 151.83 °C
From figure 12.1, assume U = 1500 W/m2.°C
Mean temperature, ∆Tm = Tsat - TB
= 151.83 – 132
= 19.83 °C
𝑄
Area required, A = 𝑈∆𝑇𝑚
= 27.11 m2
Shell-side heat transfer coefficient
𝑄
Heat flux, q = 𝐴
806.52
= 27.11
= 29.745 kW/m2
By using Mostinski (1963) correlation,
hnb = 13.55 kW/m2.°C
Critical heat flux, qc ;
qc = 361.95 kW/m2
Actual Flux, q = hnb (∆Tm)
= 268.6498 kW/m2
Since q<qc. It below critical flux
Shell diameter calculation
Use 25 mm i.d., 30 mm 0.d., 2.44 m long tubes.
Area of one tube = π25 x 10-3 x 2.44
= 0.192 m2
𝟑𝟎
Number of tubes = 𝟎.𝟏𝟗𝟐 = 157
Approximate diameter of bundle, for 1.25 square pitch
𝟏𝟓𝟕
Db = 30(𝟎.𝟐𝟏𝟓)𝟏/𝟐.𝟐𝟎𝟕
= 595 mm
A fixed tube sheet will be used for a vertical thermosyphon reboiler.
Shell diameter clearance = 14 mm.
Shell inside diameter = 595 + 14 = 609 mm
Outlet pipe diameter: take area as equal to total tube cross-sectional area
𝝅
= 157(25 x 10-3)2𝟒
= 0.077 m2
𝟎.𝟎𝟕𝟕 𝒙 𝟒
Pipe diameter = √
𝝅
= 0.31 m
Summary of Calculation
Table 6.26 shows the summary of the manual calculation for design of reboiler R-101
6.26
Reboiler specification
Area (outside) required, A
27.11 m2
Heat flux, q
29.745 kW/m2
Critical heat flux, qc
361.95 kW/m2
Actual heat flux, qa
268.6498 kW/m2
Area of 1 tube
0.192 m2
Number of tubes
157
Bundle diameter
595 mm
Shell inside diameter
609 mm
Outlet pipe diameter
0.31