Post on 29-Jun-2018
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GAS ABSORPTION
Topic Outline
• Introduction
• Basic Principles
• Applications
• Gas – Liquid Equilibrium
• Unit operation for Absorption:
a) Packed tower
b) Plate Column
• Mass Transfer between Phases
ABSORPTION UNIT
• Absorption – between gas and liquid.
• Solutes are absorbed from the gas phase into the
liquid phase.
• Absorption does not destroy the gases.
• It simply transfers the contaminated gas to the
liquid state.
• Stripping or desorption - reverse of absorption
Adsorption vs. Absorption
• Adsorption vs. Absorption– Adsorption is accumulation / adhesion of molecules at
the surface of a solid material (usually activated carbon) in contact with an air or water phase
– Absorption is dissolution of molecules within a phase, e.g., within an organic phase in contact with an air or water phase
Adsorption vs. Absorption
Basic Principles
• The type of contacting liquid chosen depends on the:
1. Solubility of solute (contaminant gases) in the chosen contacting liquid.
- pure water : NH3, acetic acid
2. Chemical reactivity between gas and liquid.
- caustic solution: acid gases, HCl & SO2
- produce a salt
Applications
1. Absorbing SO2 from the flue gases by absorption in alkaline solutions
2. Hydrogenation of edible oils in food industry
- hydrogen gas is bubbled into oil and absorbed.
3. Removal of CO2 from synthesis gases by absorbing it with hot potassium carbonate solution. (in ammonia production)
4. Absorbing dimethyl sulfide from the food processing industry
Applications
Gas-Liquid Equilibrium
• Consider the SO2-air-water system.
• An amount of gaseous SO2, air and water are put in
a closed container and shaken repeatedly at a
given temperature until equilibrium is reached.
• Samples of the gas and liquid are analyzed to
determine the partial pressure pA of SO2 in the gas
and mol fraction xA in the liquid.
Gas-Liquid Equilibrium (con’t)
• The equilibrium plot is shown in Figure 10.2-1.
• The equilibrium relation between pA in the gas phase
and xA can be expressed by a straight line Henry’s
Law equation at low concentration:
pA = H xA
Where H = Henry’s law constant (mol frac gas/ mol
frac liquid)
• The data for some common gases with water are
given in Appendix A.3 (Geankoplis, Transport
Process and Separation Process Principles, 4th ed.,
Prentice Hall)
EQUIPMENT FOR ABSORPTION UNIT
Unit Operation : PACKED TOWER
• A common apparatus used in gas absorption is the
packed tower as shown in Figure 18.1
• The device consist of:
a) cylindrical column or tower
b) gas inlet and distributing space at the bottom
c) liquid inlet and distributor at the top
d) gas & liquid outlets at the top & bottom,
respectively
e) tower packing – supported mass of inert solid
shapes
PACKED TOWER
• The liquid inlet - pure solvent or weak liquor
- is distributed over the top of packing
by the distributor
- uniformly wets the surfaces of the packings
• The distributor - is a set of perforated pipes (Fig. 18.1)
- a spray nozzles in a large towers
• The gas inlet - enter the distributing space below the packing
- flow upward in the packing countercurrent to
the flow of the liquid
PACKINGS
• The packing - provides a large area of contact between
the liquid and gas
- encourage intimates contact between the
phases
• Common dumped packings is shown in Figure 18.2.
PACKINGS• Hollow or irregular packing units – high void spaces
• Intalox saddles – the shape prevents pieces from nesting
closely together
- Increases the bed porosity
• Porosity or void fraction: 60 – 90%
• 3 principal types:
i) dumped packings, (0.25 – 3 inch)
ii) stacked packings, (2 – 8 inch)
iii) structured/ordered packings.
• Made from: plastic, metal or ceramic
Structured Packing
Ceramic Intalox Saddle Packing
Contact between liquid & gas
• Good contact between liquid & gas is the hardest to meet
esp. in large tower
• Channeling – occur at low liquid rates
- some of the packing surface dry
- chief reason for the poor performance
- severe in tower filled with stacked packings
- less severe in dumped packings
- can be minimized by having the ratio of tower
diameter to packing diameter, 8:1
Pressure Drop & Limiting Flow rates
• Figure 18.4 shows typical data for the pressure
drop in a packed tower.
• Pressure drop is due to fluid friction
• Pressure drop - common way of determining if
flooding is occuring / something else goes wrong
inside the absorber.
• The graph is plotted on logarithmic coordinates
for ΔP (inches H20/ft packing) versus the gas
flow rate, Gy (lb/ft2.h)
Loading & Flooding Point• Point K is the loading point
• Point L is the flooding point
for the given liquid flow.
• Loading point is a point
where liquid hold up starts to
increase and caused a
change in the slope of the
pressure drop
• Flooding point is a point
where the gas velocity will
result in the pressure drop
start to become almost
vertical. Liquid rapidly
accumulates, the entire
column filled with liquid.
Material Balances
The overall material balance for a countercurrent absorption process is
Lb+ Vt = Lt+ Vb
where V= vapor flow rate L= liquid flow rate t, b= top and bottom of tower, respectively
The component material balance for
species A is
LbxA,b+ VtyA,t= LtxA,t+ VbyA,b
where yA= mole fraction of A in the vapor phase xA= mole fraction of A in the liquid phase