ERT 313

BIOSEPARATION ENGINEERING

GAS ABSORPTION

Prepared by:

Pn. Hairul Nazirah Abdul Halim

Topic Outline

• Introduction

• Basic Principles

• Applications

• Gas – Liquid Equilibrium

• Unit operation for Absorption:

a) Packed tower

b) Plate Column

• Mass Transfer between Phases

Introduction

• 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

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

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)

Absorption System

• The most common design of absorption

systems:

1. Packed Bed Column / Packed Tower

2. Plate Column

Unit Operation 1: 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

FLOODING

• Occur in countercurrent flow towers

Inlet gas flow rate is so high

It interferes with the downward flow of the solvent liquid.

Cause an upward flow of the liquid through the tower

• Most absorbers are designed to operate at no

more than 70% of maximum gas velocity that can

cause flooding.

• Factors that may lead to flooding:

1. high inlet gas flow rates

2. low liquid circulation rates

3. small diameter towers

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.

Unit operation 2: PLATE COLUMN

• Plate Column absorbers distribute a contacting liquid

over plates situated one above the other.

• The contacting liquid flows downward through the

column from one plate to the other in a stepwise

fashion.

• The inlet gas rises through each plate through openings

in the plate and comes into contact with the liquid.

• Usually, a layer of foam and froth is formed above each

plate resulting from the mixing of liquid and gas.

• The gas not absorbed rises through the foam layer to

the next plate for another stage of absorption.

• Plate column absorbers result in a high removal

efficiency since there are multiple stages of contact

between liquid and gas.

• More expensive than packed bed towers.

• The advantages of plate columns are usually not

justified in small operations where a packed bed tower

will suffice.

• Plate columns have certain advantages over

packed bed towers:

a) plate columns can handle high gas flow rates

accompanied by a low liquid flowrate with little

chance of flooding.

b) little chance for channeling inside of a plate

column compared to a packed bed tower.

c) sediment build-up often can be easily removed

in plate column absorbers (packed bed towers

are harder to clean).

Principles of Absorption

• Mass Transfer between phases

• Rate of absorption

• Calculation of tower height

• Number of transfer unit

• Material Balances:

a) Packed Column

b) Plate Column

• Graphical Method: Theoretical Stages

Mass Transfer Between Phases

Two-Film Theory

• In absorption, solute from gas phase must diffuse

into liquid phase.

• The rate of diffusion in both phases affect the

overall rate of mass transfer.

• Assumption in Two-Film Theory:

a) equilibrium is assumed at the interface

b) the resistance to mass transfer in the two

phases are added to get an overall

resistance.

Mass Transfer Between Phases

• Nomenclature:

ky = mass-transfer coefficient in gas phase

kx = mass-transfer coefficient in liquid phase

Ky = Overall mass-transfer coefficient in gas

phase

Kx = Overall mass-transfer coefficient in liquid

phase

a = interfacial area per unit volume

• The rate of absorption, r per unit volume of packed column is given by any of the following equations:

where y and x refer to the mole fraction of the component being absorbed.

• The overall coefficient:

• Where m = the local slope of the equilibrium curve.

• In Eq. (18.12),

= the resistance of mass transfer in the

gas film.

= the resistance of mass transfer in the

liquid film

In Eq. (18.12):

The liquid film resistance control the rate of absorption

• when kya = kxa and m > 1.0.

• This means that any change in kxa has a nearly proportional effect on both Kya and Kxa on the rate of absorption,

• whereas a change in kya has little effect.

In Eq. (18.12):

The gas film resistance control the rate of absorption

• when kya = kxa and m << 1.0 (very small)

• Solubility of the gas is very high

• Such as absorption of HCl in water and absorption of NH3 in water

Calculation of Tower Height

Fig. 18.12 Diagram of packed absorption tower

• Consider the packed column shown in Figure 18.12.

• The cross section is S, the differential volume in height is S dZ.

• The amount absorbed in section dZ is –V dy, which equals the absorption rate times the differential volume:

• Rearrange for integration:

• The equation for column height (ZT) can be written as follows:

Number of Transfer Units (NTU)

• The integral part in Eq. (18.16) is called the number of

transfer units NTU (NOy) =

• The other part of Eq. (18.16) has the units of length and is

called the height of a transfer unit (HTU) HOy:

• Hence,

• The number of transfer units is somewhat like the number of

ideal stages (theoretical plates).

• The NTU = ideal stage if the operating line and equilibrium

line are straight and parallel as in Fig. 18.13 a.

•For straight operating and equilibrium lines:

•Where:

•The corresponding equation based on the liquid phase:

∗

∗

−=∆

−=∆

∆

∆

∆−∆=∆

bbb

aaa

a

b

abL

yyy

yyy

y

y

yyy

ln

• 4 basic types of mass transfer coefficient:

Gas Film:

Liquid Film:

Overall Gas:

Overall Liquid:

Material Balances for Packed Column

L = molal flow rate of the liquid phase

V = molal flow rate of the gas phase

x = liquid phase concentration

y = gas phase concentration

Material balances for the portion of the column above an arbitrary section (dashed line)

• Total material balance:

• Material balance on component A

Overall material equations

• Total material balance:

• Material balance on component A:

• Rearrange Eq. (18.3) gives operating-line equation:

• The operating line can be plotted on an arithmetic graph

along with the equilibrium curve as shown in Fig. 18.10.

• The operating line must lie above the equilibrium line in

order for absorption to take place.

Absorption in Plate Column

• Besides packed tower, gas

absorption can be carried out in a

column equipped with sieve trays or

other types of plates.

• Plate column is used instead of

packed column because:

a) to avoid the problem of liquid

distribution in a large diameter tower

b) to decrease the uncertainty in

scaleup

Plate Column

Material Balances for Plate Column

• A general stage in the system is

the nth stage, which is number n

counting from the entrance of the

L phase.

yn+1 = mole fraction of component A

in the V phase leaving stage

n + 1.

Ln = molal flow rate of the L phase

leaving the nth stage.

Material balances for the portion of the column above an arbitrary section (dashed line)

• Total material balance:

• Material balance on component A

Overall material balance equations

• Total material balance:

• Material balance on component A:

Graphical Methods for Two-Component Systems

• It is possible to solve many mass transfer problems

graphically for system containing only two

components.

• The operating line equation for the plate column can

be rearranged from Eq. (20.2) as below:

Graphical Methods for Two-Component Systems

• The operating line is a plot of the points xn and yn + 1 for all

the stages.

• The equilibrium line is a plot of equilibrium values of xe and

ye.

• The equilibrium data is found by experiment, by

thermodynamic calculations or from published sources.

• The position of the operating line relative to the equilibrium

line determines the direction of mass transfer and how many

stages are required for a given separation.

Ideal Contact Stages

• The ideal stage is a standard to which an actual stage

may be compared.

• If the information on stage efficiencies is available, the

no. of actual stage can be calculated.

• In an ideal stage, the V phase leaving the stage is in

equilibrium with the L phase leaving the same stage.

Determination of the number of Ideal Stages

• A simple method of determining the number of ideal

stages when there are only two components in each

phase is a graphical construction using the operating-line

diagram.

• Figure 20.5 shows the operating line and the equilibrium

curve for a typical gas absorber.

FIGURE 20.5 Operating-line diagram for gas absorber