Absorber

Topics: Sulfur dioxide, Viscosity, Gas Pages: 8 (555 words) Published: October 8, 2014
DESIGN OF AN ABSORBER
Problem statement
To design an absorber (packed tower) for the absorption of 785.15 kg/h of sodium metabisulphite (Na2S2O5) at an average temperature of 68˚C and Y atm pressure with sulphur dioxide gas counter-currently reacting with Na2SO3 rich feed. Introduction

Absorbers are vertical towers used in distillation, gas absorption and liquid-liquid extraction processes. Absorbers can be operated co-currently or counter currently. In the former, both fluids involved are fed into the unit from the same point causing them to flow along each other. In counter current operation, the liquid is fed at the top of the absorber, flows downwards and contacts with rising gas fed from beneath. Absorbers can be classified as tray towers or packed towers. In tray towers, the interfacial surface area required for contact between the two fluids is provided by stepwisely arranged plates or trays whereas in packed towers, the interfacial surface area is provided by devices called packing. (Treybal, 1981) The absorber used for this plant is a counter current packed tower. Justification

The choice of a packed tower was influenced by the following factors; 1. The liquids involved in the reaction are corrosive and packed columns provide a cheaper solution around this problem since the packing can be easily replaced when the minimum degree of corrosion is reached 2. A lower liquid hold up is achieved in packed towers than in plate towers. 3. The pressure drop per equilibrium stage (HETP) is lower for packed columns.

(A) Chemical engineering design;
(i) Component specification
(ii) Calculation of liquid and gas flow rates
(iii) Selection of packing
(iv) Hold-down plates
(v) Packing support
(vi) Determination of column height
(vii) Determination of column diameter
(viii) Height of packing
(ix) Wetting rate
(x) Absorption factor
(xi) Average gas velocity in the packing

(B) Mechanical engineering design;
(i) Material of construction
(ii) Weight of support
(iii) Critical pressure
(iv) Inner and outer diameter of column
(v) Minimum shell thickness

Table 1 .Compostion of gas inlet
Component
Mass, kg/h
Mass fraction
Amount, kmol/h
SO2
333984
0.8
5.22
N2
19263.88
0.156
0.69
O2
22695.62
0.042
0.71
Total
375943.5
0.998
6.62

Table 2. Composition of gas outlet
Component
Mass, kg/h
Mass fraction
Amount, kmol/h
SO2
4935.3
0.6731
0.08
N2
1200
0.2550
0.04
O2
1200
0.0687
0.04
Total
7335.3
0.9968
0.16

Table 3. Composition of liquid inlet
Component
Mass, kg/h
Mass fraction
Amount, kmol/h
NaHSO3
9076.4
0.3224
0.09
Na2CO3
563.12
0.02
0.01
Na2SO3
5553.51
0.1972
0.04
H2O
12963.07
0.4604
0.72
Total
28156.1
1
0.86

Table 4. composition of liquid outlet
component
mass, kg/h
mass fraction
amount, kmol/h
Na2S2O5
785.15
0.0281
0.004
NaHSO3
12249.45
0.4384
0.12
H2SO3
701.32
0.0251
0.009
Na2CO3
2235.29
0.08
0.02
Na2SO3
558.82
0.02
0.004
H2O
11421.79
0.4088
0.635
Total
27951.82
1.0004
0.79

From the tables above, the liquid and gas flow rates are specified as; Gas inlet flow rate: 6.62 Kmol/h
Gas outlet flow rate: 0.16 Kmol/h
Liquid inlet flow rate: 0.86 Kmol/h
Liquid outlet flow rate: 0.79 Kmol/h

Type of packing
The type of packing chosen for the absorber is Random Ceramic Intalox Saddle of diameter; 25mm, 38mm, 50mm, 76mm with the following features;

Table 5. chemical composition of Ceramic Intalox Saddle
Compound
Composition
SiO2
>69%
Al2O3
15-23%
SiO2+ Al2O3
>92%
Fe2O3
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